Accounting Policies [Abstract] Accounting Policies [Abstract] Billing Status, Type [Axis] Billing Status, Type Accounts Payable, Current Accounts Payable Accounts Receivable, Net, Current Accounts Receivable, Net Accrual for Environmental Loss Contingencies Total Balance at End of Period Total Balance at Beginning of Period Total Balance at End of period Accrual for Environmental Loss Contingencies, Charges to Expense for New Losses Additions Accrual for Environmental Loss Contingencies, Payments Less: Payments / Reductions Accrued Environmental Loss Contingencies, Current Less: Current Portion Accrued Environmental Loss Contingencies, Noncurrent Noncurrent Balance Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment Net Utility Plant, accumulated amortization Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Adjustments to Reconcile Net Income to Cash Provided by Operating Activities: Other Segments [Member] All Other Segments Allowance For Doubtful Accounts, Policy. Allowance For Doubtful Accounts [Policy Text Block] Allowance for Doubtful Accounts Allowance for Doubtful Accounts Receivable Allowance for Doubtful Accounts Allowance for Doubtful Accounts, Table. Allowance for Doubtful Accounts [Table Text Block] Allowance for Doubtful Accounts Included in Accounts Receivable Net Amended Credit Facility [Member] Amended Credit Facility [Member] Amended Credit Facility [Member] Amendment Flag Amendment Flag Arrangements and Non-arrangement Transactions [Domain] Arrangements and Non-arrangement Transactions [Domain] Asset Recovery Damaged Property Costs, Noncurrent Recovery amount Asset Retirement Obligations, Noncurrent Cost of removal obligation Cost of Removal Obligations Assets Identifiable Segment Assets TOTAL ASSETS Assets [Abstract] ASSETS: Assets, Current Total Current Assets Assets, Current [Abstract] Current Assets: Assets Held under Capital Leases [Member] Assets under Capital Leases Assets, Noncurrent Total Other Noncurrent Assets Assets, Noncurrent [Abstract] Other Noncurrent Assets: Award Type [Axis] Award Type Balance Sheet Location [Axis] Balance Sheet Location Balance Sheet Location [Domain] Balance Sheet Location [Domain] Basis of Accounting, Policy [Policy Text Block] Basis of Presentation Benefit Plan Costs [Member] Benefit Plan Costs [Member] Benefit Plan Costs Billed and unbilled revenue. Billed And Unbilled Revenue [Member] Billed and Unbilled Revenue Capital Expenditures Incurred but Not yet Paid Capital Expenditures Included in Accounts Payable Capitalization, Long-term Debt and Equity Total Capitalization Capitalization, Long-term Debt and Equity [Abstract] Capitalization: Capital Lease Obligations Capital lease obligation, total capitalized cost Amount of Lease Obligations Recorded on Consolidated Balance Sheets Total Capital Lease Obligations Capital Lease Obligations [Abstract] Capital Lease Obligations: Capital Lease Obligations, Current Capital lease obligation, current Other Current Liabilities (current portion) Capital Lease Obligations Incurred Payments on Capital Leases Capital Lease Obligations, Noncurrent Capital lease obligation, noncurrent Other Noncurrent Liabilities (long-term portion) Capital Leases, Future Minimum Payments Due Total Payments Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract] Capital lease Capital Leases, Future Minimum Payments Due, Next Twelve Months 2019 Capital Leases, Future Minimum Payments Due in Five Years 2023 Capital Leases, Future Minimum Payments Due in Four Years 2022 Capital Leases, Future Minimum Payments Due in Three Years 2021 Capital Leases, Future Minimum Payments Due in Two Years 2020 Capital Leases, Future Minimum Payments Due Thereafter 2024-2028 Capital Leases, Future Minimum Payments, Interest Included in Payments Less: Interest Capital Leases, Future Minimum Payments, Remainder of Fiscal Year Rest of 2019 Capital Leases Termination Date Capital Leases Termination Date Capital lease obligation, maturity period Cash and Cash Equivalents, at Carrying Value Cash and Cash Equivalents Cash and Cash Equivalents, Policy [Policy Text Block] Cash and Cash Equivalents Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Including Disposal Group and Discontinued Operations Cash and Cash Equivalents at End of Period Cash and Cash Equivalents at Beginning of Period Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect Net (Decrease) Increase in Cash and Cash Equivalents Cash Deposits Cash Deposits Cash Deposits Cash Equivalents, Maturity Period Cash Equivalents, Maturity Period Cash equivalents maturity period The net change during the reporting period in deferred regulatory and other charges. Change In Deferred Regulatory And Other Charges Deferred Regulatory and Other Charges Class of Stock [Line Items] Class of Stock [Line Items] Commercial and industrial. Commercial and Industrial [Member] C&I Commitments and Contingencies Commitments and Contingencies (Notes 6 & 7) Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Disclosure [Abstract] Common Stock, Dividends, Per Share, Declared Dividend Amount Common Stock Equity Common Stock Equity Ending Balance Beginning Balance Total Common Stock Equity Common Stock Including Additional Paid in Capital [Member] Common Equity Common Stock [Member] Common Stock Common Stock, Shares Authorized Common Stock Authorized Common Stock, Shares, Outstanding Common Equity Outstanding Common stock, shares outstanding Common Stocks, Including Additional Paid in Capital Common Equity (Authorized: 25,000,000 and Outstanding:14,916,044, 14,860,123 and 14,876,955 Shares) Retirement Benefits [Abstract] Retirement Benefits [Abstract] Components of gas operating revenue and electric operating revenue. Components of Gas Operating Revenue and Electric Operating Revenue [Table Text Block] Components of Gas and Electric Operating Revenue Contracts pro rata share percentage. Contracts Pro Rata Share Percentage Pro rata percentage share of contracts Contractual Obligation [Line Items] Contractual Obligation [Line Items] Contractual Obligation [Line Items] Contractual Obligations [Member] Contractual Obligations [Member] Energy Supply & Other Rate Adjustment Mechanisms Contractual Obligation [Table] Contractual Obligation [Table] Contractual Obligation [Table] Cost of Goods and Services Sold Operating Expenses Credit Facility [Axis] Credit Facility [Axis] Credit Facility [Domain] Credit Facility [Domain] Credit facility. Credit Facility [Member] Credit Facility Current Federal Tax Expense (Benefit) Reduction of federal tax decrease Current Fiscal Year End Date Current Fiscal Year End Date Debt Disclosure [Abstract] Debt Disclosure [Abstract] Debt Disclosure [Text Block] DEBT AND FINANCING ARRANGEMENTS Debt Instrument [Axis] Debt Instrument Debt Instrument, Basis Spread on Variable Rate Credit facility, daily fluctuating rate of interest Long-term Debt, Gross Total Long-Term Debt Debt Instrument, Covenant Description Percentage of capitalization Debt Instrument, Face Amount Long-term debt, aggregate principal amount Debt Instrument, Interest Rate, Stated Percentage Long-term debt, stated interest rate Stated interest rate Debt Instrument [Line Items] Debt Instrument [Line Items] Debt Instrument, Maturity Date Debt instrument due date Long-term debt, maturity date Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Deferred Compensation Plan [Member] Deferred Compensation Plan [Member] Deferred Compensation Plan [Member] Debt Issuance Costs, Noncurrent, Net Issuance of long-term debt Deferred Income Tax Expense (Benefit) Deferred Tax Provision Deferred income tax provision. Deferred Income Tax Provision Tax provision Deferred Revenue Arrangement, by Type [Table] Deferred Revenue Arrangement, by Type [Table] Deferred Revenue Arrangement [Line Items] Deferred Revenue Arrangement [Line Items] Deferred Tax Assets, Operating Loss Carryforwards Net Operating Loss Carryforwards Deferred Tax Assets, Operating Loss Carryforwards, Domestic Deferred tax assets, operating loss carryforwards, federal Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax Alternative minimum tax credit carryforwards Deferred Tax Liabilities, Net, Noncurrent Deferred Income Taxes, Net Defined Benefit Plan, Amortization of Gain (Loss) Actuarial Loss Amortization Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Prior Service Cost Amortization Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate Discount Rate Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets Expected Long-term rate of return on plan assets Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase Rate of Compensation Increase Defined Benefit Plan, Benefit Obligation, Benefits Paid Benefit payments under SERP Plan Amounts of pension and other post retirement benefit costs capitalized and deferred. Defined Benefit Plan, Capitalized and Deferred Amounts Capitalized and Deferred Defined Benefit Plan, Plan Assets, Contributions by Employer Contributions to benefit plan Defined Benefit Plan Disclosure [Line Items] Defined Benefit Plan Disclosure [Line Items] Defined Benefit Plan, Expected Future Benefit Payment, Remainder of Fiscal Year Expected additional benefit payments for the remainder of 2018 Defined Benefit Plan, Expected Return (Loss) on Plan Assets Expected Return on Plan Assets The gross amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain or loss, prior service cost or credit, transition asset or obligation, and gain or loss due to settlements or curtailments). Defined Benefit Plan, Gross Periodic Benefit Cost Sub-total Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Next Fiscal Year Health Care Cost Trend Rate Assumed for Next Year Defined Benefit Plan, Interest Cost Interest Cost Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Net Periodic Benefit Cost Recognized Defined Benefit Plan, Service Cost Service Cost Defined Benefit Plan, Ultimate Health Care Cost Trend Rate Ultimate Health Care Cost Trend Rate Defined Benefit Plan, Year Health Care Cost Trend Rate Reaches Ultimate Trend Rate Year that Ultimate Health Care Cost Trend Rate is reached Derivatives, Policy [Policy Text Block] Derivatives Disclosure of Dividends Declared and Paid. Disclosure of Dividends Declared and Paid [Text Block] DIVIDENDS DECLARED PER SHARE Disposal Group Including Discontinued Operations Tax Effect Of Gain On Divestiture Disposal Group Including Discontinued Operations Tax Effect Of Gain On Divestiture Provision for income taxes Divestiture [Member] Divestiture [Member] Power Supply Contract Divestitures Divestitures Policy [Policy Text Block] Divestitures Policy [Policy Text Block] Divestiture of Non-Regulated Business Subsidiary Dividend and Distribution Reinvestment and Share Purchase Plan [Member] Dividend and Distribution Reinvestment and Share Purchase Plan [Member] Dividend and Distribution Reinvestment and Share Purchase Plan Dividends Payable, Date to be Paid Date Paid (Payable) Dividends [Axis] Dividends Dividends, Common Stock Dividends on Common Shares Dividends Declared [Table Text Block] Schedule of Dividends Declared Dividends [Domain] Dividends [Domain] Dividends Payable, Date Declared Declaration Date Dividends Payable, Date of Record Shareholder of Record Date Dividends Payable [Line Items] Dividends Payable [Line Items] Dividends Payable [Table] Dividends Payable [Table] Dividends, Preferred Stock Dividend declared Document and Entity Information [Abstract] Document and Entity Information [Abstract] Document and Entity Information [Abstract] Document Fiscal Period Focus Document Fiscal Period Focus Document Fiscal Year Focus Document Fiscal Year Focus Document Information [Line Items] Document Information [Line Items] Document Information [Table] Document Information [Table] Document Period End Date Document Period End Date Document Type Document Type Domestic Country and State And Local Jurisdiction [Member] Domestic Country and State And Local Jurisdiction [Member] Federal, Maine, Massachusetts, and New Hampshire Tax Authorities Earnings Per Share, Basic and Diluted Net Income Per Common Share (Basic and Diluted) Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent Corporate federal income tax Corporate income tax rate Electric [Member] Electric [Member] Electric Electric rate payers. Electric Rate Payers [Member] Electric Ratepayers Electric Utilities [Member] Electric Utilities [Member] Deferred Storm Charges Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition Share compensation recognition period Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Stock Options Unrecognized share based compensation Energy [Axis] Energy [Axis] Energy [Domain] Energy [Domain] Total amount of obligations resulting from entry into energy supply contracts. Energy Marketing Contract Liabilities Total Energy Supply Obligations Energy Marketing Contract Liabilities, Current Energy Supply Obligations Energy Supply Obligations-Current Energy Marketing Contract Liabilities, Noncurrent Power Supply Contract Divestitures, Noncurrent Energy Related Inventory, Natural Gas in Storage Natural gas storage inventory Entity Central Index Key Entity Central Index Key Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Entity [Domain] Entity [Domain] Entity Emerging Growth Company Entity Emerging Growth Company Entity Filer Category Entity Filer Category Entity Registrant Name Entity Registrant Name Entity Small Business Entity Small Business Environmental and Rate Case Costs and Other Expenditures [Member] Environmental and Rate Case Costs and Other Expenditures [Member] Environmental and Rate Case Costs and Other Expenditures Environmental Exit Cost [Line Items] Environmental Exit Cost [Line Items] Environmental Loss Contingency Disclosure [Text Block] ENVIRONMENTAL MATTERS Environmental Matters [Member] Environmental Matters [Member] Environmental Matters Environmental Remediation Obligations [Abstract] Environmental Remediation Obligations [Abstract] Environmental Restoration Costs [Member] Environmental Restoration Costs Equity [Abstract] Equity [Abstract] Equity Component [Domain] Equity Component [Domain] Equity Funds [Member] Equity Funds Estimated flow back period. Estimated flow back period Average Rate Assumption Method estimated flow back period Facility Power Capacity Facility Power Capacity Power generation facility Fair Value Measurement Inputs and Valuation Techniques [Line Items] Fair Value Measurement Inputs and Valuation Techniques [Line Items] Fair Value Measurement Inputs and Valuation Techniques [Table] Fair Value Measurement Inputs and Valuation Techniques [Table] Fair Value Hierarchy and NAV [Axis] Fair Value, Hierarchy Liability Class [Axis] Liability Class [Axis] Fair Value, Inputs, Level 1 [Member] Fair Value, Inputs, Level 1 Fair Value, Inputs, Level 2 [Member] Fair Value, Inputs, Level 2 Fair Value by Liability Class [Domain] Fair Value by Liability Class [Domain] Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Fair Value of Restricted Stock Units to be settled in cash. Fair Value of Restricted Stock Units Settled in Cash Fair value of liabilities associated with fully vested RSUs that will be settled in cash Federal Home Loan Banks [Abstract] Federal Home Loan Banks [Abstract] Financing Arrangements [Member] Financing Arrangements [Member] Financing Arrangements First Mortgage Bonds Eight Point Four Nine Percent Senior Secured Notes Due October Fourteen Twenty Twenty Four [Member] First Mortgage Bonds Eight Point Four Nine Percent Senior Secured Notes Due October Fourteen Twenty Twenty Four [Member] First Mortgage Bonds 8.49% Senior Secured Notes, Due October 14, 2024 First Mortgage Bonds Eight Point Zero Zero Percent Senior Secured Notes Due May One Twenty Thirty One [Member] First Mortgage Bonds Eight Point Zero Zero Percent Senior Secured Notes Due May One Twenty Thirty One [Member] First Mortgage Bonds 8.00% Senior Secured Notes, Due May 1, 2031 First Mortgage Bonds Five Point Two Four Percent Senior Secured Notes Due March Two Twenty Twenty [Member] First Mortgage Bonds Five Point Two Four Percent Senior Secured Notes Due March Two Twenty Twenty [Member] First Mortgage Bonds 5.24% Senior Secured Notes, Due March 2, 2020 First Mortgage Bonds Four Point Eighteen Percent Senior Secured Notes Due November Thirty Twenty Forty Eight [Member] First Mortgage Bonds Four Point Eighteen Percent Senior Secured Notes Due November Thirty Twenty Forty Eight [Member] First Mortgage Bonds 4.18% Senior Secured Notes Due November 30, 2048 First mortgage bonds four point one eight percent due november thirty twenty forty eight. First Mortgage Bonds Four Point One Eight Percent Due November Thirty Twenty Forty Eight [Member] 4.18% First Mortgage Bonds, Due November 30, 2048 First Mortgage Bonds Six Point Nine Six Percent Senior Secured Notes Due September One Twenty Twenty Eight [Member] First Mortgage Bonds Six Point Nine Six Percent Senior Secured Notes Due September One Twenty Twenty Eight [Member] First Mortgage Bonds 6.96% Senior Secured Notes, Due September 1, 2028 First Mortgage Bonds Six Point Three Two Percent Senior Secured Notes Due September Fifteen Twenty Thirty Six [Member] First Mortgage Bonds Six Point Three Two Percent Senior Secured Notes Due September Fifteen Twenty Thirty Six [Member] First Mortgage Bonds 6.32% Senior Secured Notes, Due September 15, 2036 Fitchburg Gas and Electric Light Company [Member] Fitchburg Gas and Electric Light Company [Member] Fitchburg Gas and Electric Light Company Fixed Income Funds [Member] Fixed Income Funds Gain (Loss) on Disposition of Business Gain on Divestiture, Net (See Note 1)