Postemployment Benefits [Abstract] PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS Pension and Other Postretirement Benefits Disclosure [Text Block] Fair Value Disclosures [Abstract] Fair Value, by Balance Sheet Grouping Fair Value, by Balance Sheet Grouping [Table Text Block] Regulated Operations [Abstract] PSNH GENERATION ASSET SALE Disposal Groups, Including Discontinued Operations, Disclosure [Text Block] Derivative Instruments and Hedging Activities Disclosure [Abstract] Derivative Instruments and Hedging Activities Disclosures [Table] Derivative Instruments and Hedging Activities Disclosures [Table] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Minimum [Member] Minimum [Member] Maximum [Member] Maximum [Member] Fair Value, Hierarchy [Axis] Fair Value, Hierarchy [Axis] Fair Value Hierarchy [Domain] Fair Value Hierarchy [Domain] Fair Value, Inputs, Level 3 [Member] Fair Value, Inputs, Level 3 [Member] Derivative Instruments and Hedging Activities Disclosures [Line Items] Derivative Instruments and Hedging Activities Disclosures [Line Items] Percentage of exit price premiums related to derivative contracts Percentage of Exit Price Premiums Percentage of Exit Price Premiums Investments, Debt and Equity Securities [Abstract] Schedule of Available-for-sale Securities Reconciliation Schedule of Available-for-sale Securities Reconciliation [Table Text Block] Investments Classified by Contractual Maturity Date Investments Classified by Contractual Maturity Date [Table Text Block] Fair Value on a Recurring Basis Fair Value Hierarchy [Table Text Block] Fair Value Hierarchy [Table Text Block] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] The Connecticut Light And Power Company [Member] The Connecticut Light And Power Company [Member] NSTAR Electric Company [Member] NSTAR Electric Company [Member] Derivative Instruments and Hedging Activities Disclosures [Line Items] Percentage of costs or benefits borne under capacity-related contracts Percentage of Costs or Benefits Borne Under Capacity-Related Contracts Percentage of Costs or Benefits Borne Under Capacity-Related Contracts Percentage of costs or benefits borne under capacity-related contracts by United Illuminated Company Percentage of Costs or Benefits Borne Under Capacity-Related Contracts by United Illuminated Company Percentage of Costs or Benefits Borne Under Capacity-Related Contracts by United Illuminated Company Amount of power to be purchased under capacity-related contract (up to) Amount of Capacity Provided Under Capacity Contract Amount of Capacity Provided Under Capacity Contract Amount of energy to be purchased under renewable energy contract (in mwh) Amount of Energy to be Purchased Under Renewable Energy Contract Amount of Energy to be Purchased Under Renewable Energy Contract Amount of natural gas to be purchased under futures contracts Amount of Natural Gas To Be Purchased Under Futures Contracts Amount of Natural Gas To Be Purchased Under Futures Contracts Losses deferred as regulatory costs Derivative Instruments Gain (Loss) Recognized In Regulatory Asset Liability Derivative Instruments Gain (Loss) Recognized In Regulatory Asset Liability Debt Disclosure [Abstract] Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] 2007 Series A First and Refunding Mortgage Bonds [Member] 2007 Series A First and Refunding Mortgage Bonds [Member] 2007 Series A First and Refunding Mortgage Bonds [Member] Long-term Debt, Type [Axis] Long-term Debt, Type [Axis] Long-term Debt, Type [Domain] Long-term Debt, Type [Domain] Senior Notes [Member] Senior Notes [Member] Debt Instrument [Line Items] Debt Instrument [Line Items] Repayments of debt Repayments of Long-term Debt Interest rate stated percentage Debt Instrument, Interest Rate, Stated Percentage Equity [Abstract] Schedule of Stock by Class [Table] Schedule of Stock by Class [Table] Class of Stock [Line Items] Class of Stock [Line Items] Net income attributable to noncontrolling interests Dividends, Preferred Stock, Stock Noncontrolling Interest - Preferred Stock of Subsidiaries Redeemable Noncontrolling Interest, Equity, Preferred, Carrying Amount Commitments and Contingencies Disclosure [Abstract] Loss Contingencies [Table] Loss Contingencies [Table] Guarantor Obligations, Nature [Axis] Guarantor Obligations, Nature [Axis] Guarantor Obligations, Nature [Domain] Guarantor Obligations, Nature [Domain] Access Northeast Project Capital Contributions Guarantee Expiring In 2021 [Member] Access Northeast Project Capital Contributions Guarantee Expiring In 2021 [Member] Various Subsidiary Surety Bonds And Performance Guarantees Expiring Between 2016 And 2018 [Member] Various Subsidiary Surety Bonds And Performance Guarantees Expiring Between 2016 And 2018 [Member] Guarantee Of Rocky River Reality And Nusco Lease Payments For Real Estate And Vehicles Expiring 2019 Through 2024 [Member] Guarantee Of Rocky River Reality And Nusco Lease Payments For Real Estate And Vehicles Expiring 2019 Through 2024 [Member] Loss Contingencies [Line Items] Loss Contingencies [Line Items] Maximum Exposure Guarantor Obligations, Maximum Exposure, Undiscounted Statement of Financial Position [Abstract] Statement [Table] Statement [Table] Public Service Company Of New Hampshire [Member] Public Service Company Of New Hampshire [Member] Western Massachusetts Electric Company [Member] Western Massachusetts Electric Company [Member] Statement [Line Items] Statement [Line Items] ASSETS Assets [Abstract] Current Assets: Assets, Current [Abstract] Cash and Cash Equivalents Cash Receivables, Net Accounts Receivable, Net, Current Accounts Receivable from Affiliated Companies Accounts Receivable, Related Parties, Current Unbilled Revenues Costs in Excess of Billings, Current Taxes Receivable Income Taxes Receivable, Current Fuel, Materials, Supplies and Inventory Energy Related Inventory Materials, Supplies and Inventory Inventory, Raw Materials and Supplies, Gross Regulatory Assets Regulatory Assets, Current Prepayments and Other Current Assets Other Assets, Current Total Current Assets Assets, Current Property, Plant and Equipment, Net Public Utilities, Property, Plant and Equipment, Net Deferred Debits and Other Assets: Assets, Noncurrent [Abstract] Regulatory Assets Regulatory Assets, Noncurrent Prepaid PBOP Pension And Other Postretirement Defined Benefit Plans Assets Noncurrent Pension And Other Postretirement Defined Benefit Plans Assets Noncurrent Goodwill Goodwill Marketable Securities Marketable Securities, Noncurrent Other Long-Term Assets Other Assets, Noncurrent Total Deferred Debits and Other Assets Assets, Noncurrent Total Assets Assets LIABILITIES AND CAPITALIZATION Liabilities and Equity [Abstract] Current Liabilities: Liabilities, Current [Abstract] Notes Payable Notes Payable Notes Payable to Eversource Parent Notes Payable, Related Parties, Current Long-Term Debt – Current Portion Long-term Debt, Current Maturities Accounts Payable Accounts Payable, Current Accounts Payable to Affiliated Companies Accounts Payable, Related Parties, Current Obligations to Third Party Suppliers Accounts Payable, Other, Current Renewable Portfolio Standards Compliance Obligations Renewable Portfolio Standards Compliance Obligation The current obligation PSNH has in order to meet renewable compliance standards. Regulatory Liabilities Regulatory Liability, Current Derivative Liabilities Derivative Liability, Current Other Current Liabilities Other Liabilities, Current Total Current Liabilities Liabilities, Current Deferred Credits and Other Liabilities: Other Liabilities, Noncurrent [Abstract] Accumulated Deferred Income Taxes Deferred Tax Liabilities, Net, Noncurrent Regulatory Liabilities Regulatory Liability, Noncurrent Derivative Liabilities Derivative Liability, Noncurrent Accrued Pension and SERP Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent Other Long-Term Liabilities Other Liabilities, Noncurrent Total Deferred Credits and Other Liabilities Liabilities, Noncurrent Capitalization: Capitalization, Long-term Debt and Equity [Abstract] Long-Term Debt Long-term Debt, Excluding Current Maturities Equity: Common Shareholders' Equity: Stockholders' Equity Attributable to Parent [Abstract] Common Shares Common Stock, Value, Issued Capital Surplus, Paid In Additional Paid in Capital Retained Earnings Retained Earnings (Accumulated Deficit) Accumulated Other Comprehensive Loss Accumulated Other Comprehensive Income (Loss), Net of Tax Treasury Stock Treasury Stock, Value Common Stockholder's Equity Stockholders' Equity Attributable to Parent Total Capitalization Capitalization, Long-term Debt and Equity Total Liabilities and Capitalization Liabilities and Equity Schedule of Regulatory Assets [Table] Schedule of Regulatory Assets [Table] Regulatory Asset [Axis] Regulatory Asset [Axis] Regulatory Asset [Domain] Regulatory Asset [Domain] Benefit Costs [Member] Benefit Costs [Member] Derivative Liabilities [Member] Regulatory Assets Offsetting Derivative Liabilities [Member] The regulatory assets offsetting derivative liabilities relate to the fair value of contracts used to purchase power and other related contracts that will be collected from customers in the future. Income Taxes, Net [Member] Deferred Income Tax Charge [Member] Storm Restoration Costs [Member] Storm Restoration Costs [Member] The storm cost deferrals relate to costs incurred by the regulated company for restorations that met regulatory agency specified criteria for deferral to a major storm cost reserve. Goodwill-related [Member] Goodwill Regulatory Asset [Member] Goodwill that originated from the merger that created NSTAR in 1999. Regulatory Tracker Mechanisms [Member] Regulatory Tracker Deferrals Regulatory Assets [Member] Regulatory tracker deferrals are approved rate mechanisms that allow utilities to recover costs in specific business segments through reconcilable tracking mechanisms that are reviewed at least annually by the applicable regulatory commission. Regulatory tracker deferrals are recorded as regulatory assets if unrecovered costs are in excess of collections and are recorded as regulatory liabilities if collections are in excess of costs. Contractusl Obligations - Yankee Companies [Member] Unrecovered Contractual Obligations [Member] Represents unrecovered contractual obligations (decommissioning and plant closure costs) related to the Yankee Companies in excess of collections. Other Regulatory Assets [Member] Other Regulatory Assets [Member] Represents various remaining other regulatory assets. Regulatory Assets [Line Items] Regulatory Assets [Line Items] Total Regulatory Assets Regulatory Assets Less: Current Portion Total Long-Term Regulatory Assets Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Defined Benefit Plans and Other Postretirement Benefit Plans [Axis] Defined Benefit Plans and Other Postretirement Benefit Plans [Axis] Defined Benefit Plan and Other Postretirement Benefit Plan [Domain] Defined Benefit Plan and Other Postretirement Benefit Plan [Domain] Pension Plan [Member] Pension Plan [Member] Other Postretirement Benefit Plan [Member] Other Postretirement Benefit Plan [Member] Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Service Cost Defined Benefit Plan, Service Cost Interest Cost Defined Benefit Plan, Interest Cost Expected Return on Pension Plan Assets Defined Benefit Plan, Expected Return on Plan Assets Actuarial Loss Defined Benefit Plan, Amortization of Gains (Losses) Prior Service Cost/(Credit) Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Total Net Periodic Benefit Income Defined Benefit Plan, Net Periodic Benefit Cost Intercompany Allocations Related Intercompany Allocationsof Defined Benefit Expense Amount represents allocation of pension and other post retirement benefits expense from parent and other that are not included in pension and post-retirement benefit expense. Capitalized PBOP Income Amount Capitalized Defined Benefit Expense Amount represents the amount of defined benefit plan expense capitalized Public Utility [Axis] Public Utility [Axis] Utility Plant [Domain] Utility Plant [Domain] Electricity Generation Plant, Non-Nuclear [Member] Electricity Generation Plant, Non-Nuclear [Member] Number of state senators Number of State Senators Number of State Senators Equity return related to clean air project forgone Equity Return Related to Clean Air Project Forgone Equity Return Related to Clean Air Project Forgone Payments for restructuring Payments for Restructuring Accounts payable Accounts Payable Other current liabilities Asset retirement obligation Asset Retirement Obligation Income Statement [Abstract] Operating Revenues Revenues Operating Expenses: Operating Expenses [Abstract] Purchased Power, Fuel and Transmission Cost of Purchased Power Operations and Maintenance Utilities Operating Expense, Maintenance and Operations Depreciation Depreciation Amortization of Regulatory Assets, Net Amortization of Regulatory Asset Amortization of Regulatory Asset (Liability) Amortization of Regulatory Asset (Liability) Amortization of Regulatory Asset (Liability) Energy Efficiency Programs Energy Efficiency Programs These costs are in accordance with energy efficiency and conservation program guidelines established by regulators. Taxes Other Than Income Taxes Taxes, Miscellaneous Total Operating Expenses Operating Expenses Operating Income/(Loss) Operating Income (Loss) Interest Expense Interest Expense Other Income/(Loss), Net Nonoperating Income (Expense) Income Before Income Tax Expense Income (Loss) from Continuing Operations before Income Taxes, Domestic Income Tax Expense Income Tax Expense (Benefit) Net Income Net Income (Loss) Attributable to Parent Net Income Attributable to Noncontrolling Interests Net Income (Loss) Attributable to Noncontrolling Interest Net Income Attributable to Common Shareholders Net Income (Loss) Available to Common Stockholders, Basic Dividends Declared Per Common Share (in dollars per share) Common Stock, Dividends, Per Share, Declared Weighted Average Common Shares Outstanding: Weighted Average Number of Shares Outstanding, Diluted [Abstract] Basic (in shares) Weighted Average Number of Shares Outstanding, Basic Diluted (in shares) Weighted Average Number of Shares Outstanding, Diluted REGULATORY ACCOUNTING Public Utilities Disclosure [Text Block] Statement of Comprehensive Income [Abstract] Net Income Other Comprehensive (Loss)/Income, Net of Tax: Other Comprehensive Income (Loss), Net of Tax [Abstract] Qualified Cash Flow Hedging Instruments Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax Changes in Unrealized Gains/(Losses) on Marketable Securities Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax Changes in Funded Status of Pension, SERP and PBOP Benefit Plans Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax Net OCI Other Comprehensive Income (Loss), Net of Tax Comprehensive Income Attributable to Noncontrolling Interests Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest Comprehensive Income Attributable to Common Shareholders Comprehensive Income (Loss), Net of Tax, Attributable to Parent Schedule of Derivative Instruments in Statement of Financial Position, Fair Value Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block] Fair Value Inputs, Liabilities, Quantitative Information Fair Value Inputs, Liabilities, Quantitative Information [Table Text Block] Rollforward Of Net Derivative Asset Liabilities Valued Using Unobservable Inputs Rollforward Of Net Derivative Asset Liabilities Valued Using Unobservable Inputs [Table Text Block] Schedule of Rollforward of fair value measurements of derivatives valued using unobservable inputs. Earnings Per Share [Abstract] EARNINGS PER SHARE Earnings Per Share [Text Block] Document and Entity Information [Abstract] Document Information [Table] Document Information [Table] Document And Entity [Line Items] Document And Entity [Line Items] Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Entity Filer Category Entity Filer Category Document Type Document Type Document Period End Date Document Period End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Amendment Flag Amendment Flag Trading Symbol Trading Symbol Entity Common Stock, Shares Outstanding (in shares) Entity Common Stock, Shares Outstanding Accounting Policies [Abstract] Schedule of Equity Method Investments [Table] Schedule of Equity Method Investments [Table] Geographical [Axis] Geographical [Axis] Geographical [Domain] Geographical [Domain] Connecticut, Massachusetts and New Hampshire [Member] Connecticut, Massachusetts and New Hampshire [Member] Connecticut, Massachusetts and New Hampshire [Member] Schedule of Equity Method Investments [Line Items] Schedule of Equity Method Investments [Line Items] Number of electric and natural gas customers Number of Electric and Natural Gas Customers Number of Electric and Natural Gas Customers Number of regulated utilities Number of Regulated Utilities Number of Regulated Utilities Schedule of Regulatory Assets Schedule of Regulatory Assets [Table Text Block] Schedule of Regulatory Liabilities Schedule of Regulatory Liabilities [Table Text Block] Segment Reporting [Abstract] SEGMENT INFORMATION Segment Reporting Disclosure [Text Block] Available for sale securities subject to new accounting guidance Available for sale securities subject to new accounting guidance Amount represents the fair value of the available for sale equity securities that will be subject to the new accounting guidance requiring changes in fair value to be recorded in income Accumulated Other Comprehensive Income (Loss) [Table] Accumulated Other Comprehensive Income (Loss) [Table] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] AOCI Attributable to Parent [Member] AOCI Attributable to Parent [Member] Qualified Cash Flow Hedging Instruments [Member] Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] Unrealized Gains/(Losses) on Marketable Securities [Member] Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] Defined Benefit Plans [Member] Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] AOCI Attributable to Parent [Roll Forward] AOCI Attributable to Parent [Abstract] Balance as of Beginning of Period OCI Before Reclassifications Other Comprehensive Income (Loss), before Reclassifications, Net of Tax Amounts Reclassified from AOCL Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax Balance as of End of Period Schedule of Deferred Compensation Arrangement with Individual, Postretirement Benefits [Table] Schedule of Deferred Compensation Arrangement with Individual, Postretirement Benefits [Table] Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items] Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items] Prior service credit as a result of remeasurement Impact of Change in Accounting Estimate Spot Rate Discount Rate Represents in reduction in periodic benefit plan cost for the change to the spot rate methdology of calculating the discount rate Schedule of Accounts, Notes, Loans and Financing Receivable [Table] Schedule of Accounts, Notes, Loans and Financing Receivable [Table] Yankee Gas Services Company [Member] Yankee Gas Services Company [Member] Accounts, Notes, Loans and Financing Receivable [Line Items] Accounts, Notes, Loans and Financing Receivable [Line Items] Period of accounts receivable recoverable under financial or medical duress Period of Accounts Receivable Recoverable Under Financial or Medical Duress Period of Accounts Receivable Recoverable Under Financial or Medical Duress Schedule of Supplemental Cash Flow [Table] Schedule of Supplemental Cash Flow [Table] Schedule of Supplemental Cash Flow [Table] Schedule of Supplemental Cash Flow [Line Items] Schedule of Supplemental Cash Flow [Line Items] [Line Items] for Schedule of Supplemental Cash Flow [Table] Capital expenditures incurred but not yet paid Capital Expenditures Incurred but Not yet Paid SHORT-TERM AND LONG-TERM DEBT Debt Disclosure [Text Block] Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Number of companies that transmit hydro electricity imported from The Hydro-Quebec System in Canada Number of Companies That Transmit Hydro Electricity Imported From The Hydro-Quebec System in Canada Number of Companies That Transmit Hydro Electricity Imported From The Hydro-Quebec System in Canada Amount of natural gas transmission purchased from Enbridge by Yankee Gas and NSTAR Gas Amount of Natural Gas Transmission Purchased From Enbridge by Yankee Gas and NSTAR Gas Amount of Natural Gas Transmission Purchased From Enbridge by Yankee Gas and NSTAR Gas Number of reportable segments Number of Reportable Segments Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Event [Member] Subsequent Event [Member] Loss Contingency Nature [Axis] Loss Contingency Nature [Axis] Loss Contingency, Nature [Domain] Loss Contingency, Nature [Domain] FERC ROE First, Second and Third Complaints [Member] FERC ROE First, Second and Third Complaints [Member] FERC ROE First, Second and Third Complaints [Member] FERC ROE Complaints [Member] FERC ROE Complaints [Member] FERC ROE Complaints [Member] FERC ROE Fourth Complaint [Member] FERC ROE Fourth Complaint [Member] FERC ROE Fourth Complaint [Member] FERC ROE First Complaint [Member] FERC ROE First Complaint [Member] FERC ROE First Complaint [Member] FERC ROE Second Complaint [Member] FERC ROE Second Complaint [Member] FERC ROE Second Complaint [Member] Base ROE percentage challenged by complainants Loss Contingency, Base ROE Percentage Challenged by Complainants Loss Contingency, Percentage of Base ROE Challenged by Complainants Period of complaint Loss Contingency, Period of Complaint Loss Contingency, Period of Complaint Base ROE percentage of complaint period Percentage of Base ROE of Complaint Period Percentage of Base ROE of Complaint Period Maximum percentage cap of ROE for incentive projects Loss Contingency, Maximum percentage of ROE for incentive projects Loss Contingency, Maximum percentage of ROE for incentive projects Basis point change Loss Contingency, Basis Spread Loss Contingency, Basis Spread Loss contingency, estimate of possible earnings impact Loss Contingency, Estimate of Possible Loss Estimate of possible loss for each 10 basis point change to base ROE Estimate of possible loss for each 10 basis point change to base ROE Estimate of possible loss for each 10 basis point change to base ROE Property, Plant and Equipment [Abstract] Public Utility Property, Plant, and Equipment Public Utility Property, Plant, and Equipment [Table Text Block] PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION Property, Plant and Equipment Disclosure [Text Block] COMMON SHAREHOLDERS' EQUITY AND NONCONTROLLING INTERESTS Stockholders' Equity Note Disclosure [Text Block] Site Contingency [Table] Site Contingency [Table] Environmental Remediation Site [Axis] Environmental Remediation Site [Axis] Environmental Remediation Site [Domain] Environmental Remediation Site [Domain] MGP Site accrual [Member] MGP Site accrual [Member] Portion of environmental liability associated with MGP sites Site Contingency [Line Items] Site Contingency [Line Items] Reserve Accrual for Environmental Loss Contingencies Eversource and The Connecticut Light And Power Company [Member] Eversource and The Connecticut Light And Power Company [Member] Eversource and The Connecticut Light And Power Company [Member] Eversource and NSTAR Electric Company [Member] Eversource and NSTAR Electric Company [Member] Eversource and NSTAR Electric Company [Member] Capacity Prices (in USDPerKiloWattMonth) FairValueInputsCapacityPrices Represents capacity prices used in derivative valuations for periods not yet observable. Forward Reserve (in USDPerKiloWattMonth) FairValueInputsForwardReserve Represents forward reserve market prices used in 2 of the CfDs REC Prices (in USDPerKiloWattMonth) FairValueInputsRenewableEnergyCreditPrices Represents the price per renewable energy credit. Net Income Attributable to Common Shareholders Weighted Average Common Shares Outstanding: Weighted Average Number of Shares Outstanding Reconciliation [Abstract] Dilutive Effect (in shares) Weighted Average Number Diluted Shares Outstanding Adjustment Diluted (in shares) Basic and Diluted EPS (in dollars per share) Earnings Per Share, Basic and Diluted Derivatives, Net [Rollforward] Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward] Fair Value as of Beginning of Period Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value Net Realized/Unrealized Losses Included in Regulatory Assets and Liabilities Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain (Loss) Included In Regulatory Asset Liability Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain (Loss) Included In Regulatory Asset Liability Settlements Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements Fair Value as of End of Period Schedule of Gross Tax Receipts [Table] Schedule of Gross Tax Receipts [Table] Schedule of Gross Tax Receipts [Table] Schedule of Gross Tax Receipts [Line Items] Schedule of Gross Tax Receipts [Line Items] [Line Items] for Schedule of Gross Tax Receipts [Table] Excise and sales taxes Excise and Sales Taxes Gross Plant Public Utilities, Property, Plant and Equipment, Plant in Service Accumulated Depreciation Public Utilities, Property, Plant and Equipment, Accumulated Depreciation Net Plant Fuel Public Utilities, Property, Plant and Equipment, Fuel Materials and Supplies Public Utilities Property Plant And Equipment Materials and Supplies Public Utilities Property Plant And Equipment Materials and Supplies Emission Allowances Total Generation Assets Public Utilities Property Plant And Equipment, Gross Public Utilities Property Plant And Equipment, Gross Antidilutive share awards excluded from computation (in shares) Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount ACCUMULATED OTHER COMPREHENSIVE INCOME/(LOSS) Comprehensive Income (Loss) Note [Text Block] DERIVATIVE INSTRUMENTS Derivative Instruments and Hedging Activities Disclosure [Text Block] Schedule of Earnings Per Share, Basic and Diluted Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Fair Value Measurements, Recurring and Nonrecurring [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Measurement Basis [Axis] Measurement Basis [Axis] Fair Value Measurement [Domain] Fair Value Measurement [Domain] Portion at Fair Value Measurement [Member] Portion at Fair Value Measurement [Member] Carrying Amount Reported Value Measurement [Member] Fair Value Estimate of Fair Value Measurement [Member] Fair Value, Inputs, Level 2 [Member] Fair Value, Inputs, Level 2 [Member] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Preferred Stock Not Subject to Mandatory Redemption Long-Term Debt Long-term Debt Schedule of Regulatory Liabilities [Table] Schedule of Regulatory Liabilities [Table] Regulatory Liability [Axis] Regulatory Liability [Axis] Regulatory Liability [Domain] Regulatory Liability [Domain] Cost of Removal [Member] Cost Of Removal [Member] The regulated companies currently recover amounts in rates for future costs of removal of plant assets over the lives of the assets. Regulatory Tracker Mechanisms [Member] Regulatory Tracker Deferrals Regulatory Liabilities [Member] Regulatory tracker deferrals are approved rate mechanisms that allow utilities to recover costs in specific business segments through reconcilable tracking mechanisms that are reviewed at least annually by the applicable regulatory commission. Regulatory tracker deferrals are recorded as regulatory assets if unrecovered costs are in excess of collections and are recorded as regulatory liabilities if collections are in excess of costs. AFUDC - Transmission [Member] AFUDC Transmission Incentive [Member] AFUDC is recorded on 100 percent of CL&P's and WMECO's CWIP for their NEEWS projects, all of which is being reserved as a regulatory liability to reflect current rate base recovery for 100 percent of the CWIP as a result of FERC-approved transmission incentives. Other Regulatory Liabilities [Member] Other Regulatory Liabilities [Member] Represents various remaining other regulatory liabilities. Regulatory Liabilities [Line Items] Regulatory Liabilities [Line Items] Total Regulatory Liabilities Regulatory Liabilities Less: Current Portion Total Long-Term Regulatory Liabilities Basis of Presentation Basis of Accounting, Policy [Policy Text Block] Accounting Standards New Accounting Pronouncements, Policy [Policy Text Block] Provision for Uncollectible Accounts Receivables, Policy [Policy Text Block] Fair Value Measurements Fair Value of Financial Instruments, Policy [Policy Text Block] Other Income, Net Other Income, Net [Policy Text Block] Other Income, Net [Policy Text Block] Other Taxes Income Tax, Policy [Policy Text Block] Consolidation Items [Axis] Consolidation Items [Axis] Consolidation Items [Domain] Consolidation Items [Domain] Operating Segments [Member] Operating Segments [Member] Eliminations [Member] Intersegment Eliminations [Member] Segments [Axis] Segments [Axis] Segments [Domain] Segments [Domain] Electric Distribution [Member] Electric Distribution [Member] Natural Gas Distribution [Member] Natural Gas Distribution [Member] Electric Transmission [Member] Electric Transmission [Member] Other [Member] Other Segment [Member] Operating Revenues Depreciation and Amortization Depreciation, Depletion and Amortization Other Operating Expenses Utilities Operating Expense, Other Interest Expense Other Income/(Loss), Net Cash Flows Used for Investments in Plant Payments to Acquire Property, Plant, and Equipment Assets Fair Value By Asset Class Axis [Axis] Fair Value By Asset Class Axis [Axis] Fair value information by class of asset. Fair Value, Inputs, Level 1 [Member] Fair Value, Inputs, Level 1 [Member] Major Types of Debt and Equity Securities [Axis] Major Types of Debt and Equity Securities [Axis] Major Types of Debt and Equity Securities [Domain] Major Types of Debt and Equity Securities [Domain] Mutual Funds and Equities [Member] Mutual Funds [Member] Represents investments in mutual and exchange traded funds Money Market Funds [Member] Cash and Cash Equivalents [Member] U.S. Government Issued Debt Securities (Agency and Treasury) [Member] US Government Debt Securities [Member] Corporate Debt Securities [Member] Corporate Bond Securities [Member] Asset-Backed Debt Securities [Member] Asset-backed Securities [Member] Municipal Bonds [Member] Municipal Bonds [Member] Other Fixed Income Securities [Member] Other Debt Obligations [Member] Marketable securities recorded at fair value on a recurring basis Investments, Fair Value Disclosure Public Utility, Property, Plant and Equipment [Table] Public Utility, Property, Plant and Equipment [Table] Public Utility, Property, Plant and Equipment [Line Items] Public Utility, Property, Plant and Equipment [Line Items] Distribution - Electric Public Utilities, Property, Plant and Equipment, Distribution Distribution - Natural Gas Public Utilities Property Plant And Equipment Distribution Natural Gas Period end amount of property, plant and equipment (PPE) related to utility natural gas distribution. Transmission Public Utilities, Property, Plant and Equipment, Transmission Generation Public Utilities, Property, Plant and Equipment, Generation or Processing Electric and Natural Gas Utility Public Utilities Property Plant And Equipment Electric And Natural Gas Utility Total Period end amount of property, plant and equipment (PPE) related to utility Other Public Utilities, Property, Plant and Equipment, Other Property, Plant and Equipment Property, Plant and Equipment, Gross Less: Accumulated Depreciation Depreciation, Depletion and Amortization [Abstract] Electric and Natural Gas Utility Electric And Natural Gas Utility Accumulated Depreciation Period end book value of accumulated depreciation on distribution related property, plant and equipment (PPE) that is owned by the regulated operations of the public utility." Other Other Accumulated Depreciation Period end book value of accumulated depreciation on other property, plant and equipment (PPE) owned by the regulated operations of the public utility. Total Accumulated Depreciation Property, Plant and Equipment, Net Property Plant And Equipment Net Excluding Construction Work In Progress Period end amount of property, plant and equipment (PPE) related to utility distribution. Construction Work in Progress Public Utilities, Property, Plant and Equipment, Construction Work in Progress Scenario [Axis] Scenario [Axis] Scenario, Unspecified [Domain] Scenario, Unspecified [Domain] Scenario, Forecast [Member] Scenario, Forecast [Member] FERC ROE Third Complaint [Member] FERC ROE Third Complaint [Member] FERC ROE Third Complaint [Member] Original Base ROE Authorized by FERC at Time of Complaint (in percentage) Loss Contingency, Original Base ROE Authorized by FERC at Time of Complaint Loss Contingency, Original Base ROE Authorized by FERC at Time of Complaint Base ROE Subsequently Authorized by FERC for first complaint period and prospectively from October 16, 2014 (in percentage) Loss Contingency, Base ROE Subsequently Authorized by FERC for First Complaint Period and Prospectively From October 2014 Loss Contingency, Base ROE Subsequently Authorized by FERC for First Complaint Period and Prospectively From October 2014 Reserve (Pre-Tax and Excluding Interest) as of March 31, 2017 (in millions) FERC ALJ Recommendation of base ROE on Second and Third Complaints (Issued March 22, 2016) (in percentage) Loss Contingency, FERC ALJ Recommended base ROE percentage Loss Contingency, Percentage of Base ROE Zone of Reasonableness Percentage of maximum ROE for any incentive project Loss Contingency, Percentage of ROE for Any Incentive Project Loss Contingency, Percentage of ROE for Any Incentive Project Customer Refund Payments Customer Refundable Fees, Refund Payments Schedule of Segment Reporting Information, by Segment Schedule of Segment Reporting Information, by Segment [Table Text Block] COMMON SHARES Statement of Cash Flows [Abstract] Operating Activities: Net Cash Provided by (Used in) Operating Activities [Abstract] Adjustments to Reconcile Net Income to Net Cash Flows Provided by Operating Activities: Adjustments, Noncash Items, to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Deferred Income Taxes Deferred Income Tax Expense (Benefit) Pension, SERP and PBOP Expense, Net Pension and Other Postretirement Benefit Expense Pension and PBOP Contributions Pension and Other Postretirement Benefit Contributions Pension and PBOP Contributions, Net of Pension, SERP and PBOP Expense Pension, SERP and PBOP Expense, Net of Contributions Pension, SERP and PBOP Expense, Net of Contributions Regulatory Over/(Under) Recoveries, Net Increase (Decrease) in Regulatory Assets and Liabilities Amortization of Regulatory Assets, Net Amortization of Regulatory (Liabilities)/Assets, Net Other Other Noncash Income (Expense) Changes in Current Assets and Liabilities: Increase (Decrease) in Operating Capital [Abstract] Receivables and Unbilled Revenues, Net Increase (Decrease) in Receivables Fuel, Materials, Supplies and Inventory Increase (Decrease) in Inventories Taxes Receivable/Accrued, Net Increase (Decrease) in Income Taxes Receivable Accounts Payable Increase (Decrease) in Accounts Payable Other Current Assets and Liabilities, Net Increase (Decrease) in Other Operating Assets and Liabilities, Net Net Cash Flows Provided by Operating Activities Net Cash Provided by (Used in) Operating Activities Investing Activities: Net Cash Provided by (Used in) Investing Activities [Abstract] Investments in Property, Plant and Equipment Proceeds from Sales of Marketable Securities Proceeds from Sale and Maturity of Marketable Securities Purchases of Marketable Securities Payments to Acquire Marketable Securities Proceeds from the Sale of Property, Plant and Equipment Proceeds from Sale of Property, Plant, and Equipment Other Investing Activities Payments for (Proceeds from) Other Investing Activities Net Cash Flows Used in Investing Activities Net Cash Provided by (Used in) Investing Activities Financing Activities: Net Cash Provided by (Used in) Financing Activities [Abstract] Cash Dividends on Common Shares Payments of Ordinary Dividends, Common Stock Capital Contributions from Eversource Parent Proceeds from Contributions from Parent Cash Dividends on Preferred Stock Payments of Ordinary Dividends, Preferred Stock and Preference Stock Decrease in Notes Payable to Eversource Parent Increase (Decrease) in Due to Related Parties Decrease in Notes Payable Proceeds from (Repayments of) Short-term Debt Issuance of Long-Term Debt Proceeds from Issuance of Long-term Debt Retirements of Long-Term Debt Other Financing Activities Proceeds from (Payments for) Other Financing Activities Net Cash Flows Provided by/(Used in) Financing Activities Net Cash Provided by (Used in) Financing Activities Net Increase in Cash and Cash Equivalents Cash and Cash Equivalents, Period Increase (Decrease) Cash and Cash Equivalents - Beginning of Period Cash and Cash Equivalents - End of Period Schedule of Generation Assets and Liabilities Disclosure of Long Lived Assets Held-for-sale [Table Text Block] Amortized Cost Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract] Less than one year Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis One to five years Available-for-sale Securities, Debt Maturities, Year Two Through Five, Amortized Cost Basis Six to ten years Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Amortized Cost Basis Greater than ten years Available-for-sale Securities, Debt Maturities, after Ten Years, Amortized Cost Basis Total Debt Securities Available-for-sale Debt Securities, Amortized Cost Basis Fair Value Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract] Less than one year Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value One to five years Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value Six to ten years Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value Greater than ten years Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value Total Debt Securities Available-for-sale Securities, Debt Securities MARKETABLE SECURITIES Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] Total Provision for Uncollectible Accounts Allowance for Doubtful Accounts Receivable, Current Uncollectible Hardship Uncollectible Hardship Accounts Receivable Reserve An allowance for uncollectible hardship accounts receivable which is allowed to be deferred and recovered in rates by the applicable regulatory commission. The corresponding bad debt expense is recorded as regulatory assets as these amounts are probable of recovery. Per Value (in dollars per share) Common Stock, Par or Stated Value Per Share Authorized (in shares) Common Stock, Shares Authorized Issued (in shares) Common Stock, Shares, Issued Schedule of Accumulated Other Comprehensive Income (Loss) Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Number of Sites Environmental Site Quantity Number of environmental sites by NU Entity in the remediation or long-term monitoring phase Balance Sheet Location [Axis] Balance Sheet Location [Axis] Balance Sheet Location [Domain] Balance Sheet Location [Domain] Current Derivative Assets [Member] Prepaid Expenses and Other Current Assets [Member] Long-term Derivative Assets [Member] Other Noncurrent Assets [Member] Current Derivative Liabilities [Member] Other Current Liabilities [Member] Long-Term Derivative Liabilities [Member] Other Noncurrent Liabilities [Member] Derivative Assets (Liabilities), at Fair Value, Net, by Balance Sheet Classification [Abstract] Derivative Assets (Liabilities), at Fair Value, Net, by Balance Sheet Classification [Abstract] Commodity Supply and Price Risk Management Derivative Asset, Fair Value, Gross Asset Netting Derivative Asset, Fair Value, Gross Liability Net Amount Recorded as a Derivative Derivative Asset, Fair Value, Amount Not Offset Against Collateral Current and Long-Term Derivative Liabilities Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] Commodity Supply and Price Risk Management Derivative Liability, Fair Value, Gross Liability Netting Derivative Liability, Fair Value, Gross Asset Net Amount Recorded as a Derivative Derivative Liability, Fair Value, Amount Not Offset Against Collateral FAIR VALUE OF FINANCIAL INSTRUMENTS Fair Value Disclosures [Text Block] Trading Securities [Table] Trading Securities [Table] Income Statement Location [Axis] Income Statement Location [Axis] Income Statement Location [Domain] Income Statement Location [Domain] Other Income [Member] Other Income [Member] Schedule of Trading Securities and Other Trading Assets [Line Items] Schedule of Trading Securities and Other Trading Assets [Line Items] Fair value of securities under fair value option Fair Value Of Securities Under Fair Value Option Represents the fair value of equity securities accounted for under the fair value option. Net gain (loss) under fair value option Fair Value, Option, Changes in Fair Value, Gain (Loss) Schedule of Guarantor Obligations [Table] Schedule of Guarantor Obligations [Table] Access Northeast [Member] Access Northeast [Member] Access Northeast [Member] Guarantee Of Financial Obligations Of Npt [Member] Guarantee Of Financial Obligations Of Npt [Member] Guarantee Of Npt Letters Of Credit [Member] Guarantee Of Npt Letters Of Credit [Member] Guarantor Obligations [Line Items] Guarantor Obligations [Line Items] Guarantor obligations, maximum exposure (up to) Amount of debt authorized (up to) Amount of Debt Authorized Amount of Debt Authorized SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Significant Accounting Policies [Text Block] Schedule of Net Benefit Costs Schedule of Net Benefit Costs [Table Text Block] COMMITMENTS AND CONTINGENCIES Commitments and Contingencies Disclosure [Text Block] Eversource Parent [Member] Parent Company [Member] Series K Senior Notes Due 2022 [Member] Series K Senior Notes Due 2022 [Member] Series K Senior Notes Due 2022 [Member] 2017 Series A First and Refunding Mortgage Bonds Due 2027 [Member] 2017 Series A First and Refunding Mortgage Bonds Due 2027 [Member] 2017 Series A First and Refunding Mortgage Bonds Due 2027 [Member] Debt instrument, face amount Debt Instrument, Face Amount Debt, equity and marketable securities held in nuclear decommissioning trust Marketable Securities, Restricted, Noncurrent Schedule of Environmental Loss Contingencies by Site Schedule of Environmental Loss Contingencies by Site [Table Text Block] Schedule of Guarantor Obligations Schedule of Guarantor Obligations [Table Text Block] Schedule of Complaints and Base ROE Schedule of Complaints and Base ROE [Table Text Block] Schedule of Complaints and Base ROE [Table Text Block] Amount of regulatory costs not yet approved Amount of Regulatory Costs Not yet Approved Line of Credit Facility [Table] Line of Credit Facility [Table] Short-term Debt, Type [Axis] Short-term Debt, Type [Axis] Short-term Debt, Type [Domain] Short-term Debt, Type [Domain] Commercial Paper [Member] Commercial Paper [Member] Line of Credit [Member] Line of Credit [Member] Line of Credit Facility [Line Items] Line of Credit Facility [Line Items] Maximum borrowing capacity Line of Credit Facility, Maximum Borrowing Capacity Short-term borrowings outstanding Short-term Debt Remaining borrowing capacity Line of Credit Facility, Remaining Borrowing Capacity Weighted average interest rate (as a percentage) Short-term Debt, Weighted Average Interest Rate Notes payable to Eversource Parent Debt instrument term Debt Instrument, Term Amount outstanding during period Line of Credit Facility, Amount Outstanding During Period Line of Credit Facility, Amount Outstanding During Period Treasury stock (in shares) Treasury Stock, Shares Common stock outstanding (in shares) Common Stock, Shares, Outstanding Schedule of Stock by Class Schedule of Stock by Class [Table Text Block] Schedule of Available-for-sale Securities [Table] Schedule of Available-for-sale Securities [Table] Debt Securities [Member] Debt Securities [Member] Equity Securities [Member] Equity Securities [Member] Schedule of Available-for-sale Securities [Line Items] Schedule of Available-for-sale Securities [Line Items] Amortized Cost Available-for-sale Securities, Amortized Cost Basis Pre-Tax Unrealized Gains Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax Pre-Tax Unrealized Losses Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax Fair Value Available-for-sale Securities Schedule of Accounts, Notes, Loans and Financing Receivable Schedule of Accounts, Notes, Loans and Financing Receivable [Table Text Block] State Of Connecticut Gross Earnings Taxes Schedule of Gross Earnings Taxes [Table Text Block] Schedule of Gross Earnings Taxes [Table Text Block] Schedule of Supplemental Cash Flow Information Schedule of Cash Flow, Supplemental Disclosures [Table Text Block]