SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	6 Months Ended
Jun. 30, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation Eversource Energy is a public utility holding company primarily engaged, through its wholly-owned regulated utility subsidiaries, in the energy delivery business. Eversource Energy's wholly-owned regulated utility subsidiaries consist of CL&P, NSTAR Electric and PSNH (electric utilities), Yankee Gas and NSTAR Gas (natural gas utilities) and Aquarion (water utilities). Eversource provides energy delivery and/or water service to approximately four million electric, natural gas and water customers through eight regulated utilities in Connecticut, Massachusetts and New Hampshire. The unaudited condensed consolidated financial statements of Eversource, NSTAR Electric and PSNH include the accounts of each of their respective subsidiaries. Intercompany transactions have been eliminated in consolidation. The accompanying unaudited condensed consolidated financial statements of Eversource, NSTAR Electric and PSNH and the unaudited condensed financial statements of CL&P are herein collectively referred to as the "financial statements." The combined notes to the financial statements have been prepared pursuant to the rules and regulations of the SEC. Certain information and footnote disclosures included in annual financial statements prepared in accordance with GAAP have been omitted pursuant to such rules and regulations. The accompanying financial statements should be read in conjunction with the Combined Notes to Financial Statements included in Item 8, "Financial Statements and Supplementary Data," of the Eversource 2018 Form 10-K, which was filed with the SEC on February 26, 2019. The preparation of the financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. The financial statements contain, in the opinion of management, all adjustments (including normal, recurring adjustments) necessary to present fairly Eversource's, CL&P's, NSTAR Electric's and PSNH's financial position as of June 30, 2019 and December 31, 2018, and the results of operations, comprehensive income and common shareholders' equity for the three and six months ended June 30, 2019 and 2018, and the cash flows for the six months ended June 30, 2019 and 2018. The results of operations and comprehensive income for the three and six months ended June 30, 2019 and 2018 and the cash flows for the six months ended June 30, 2019 and 2018 are not necessarily indicative of the results expected for a full year. Eversource consolidates CYAPC and YAEC because CL&P's, NSTAR Electric's and PSNH's combined ownership and voting interests in each of these entities is greater than 50 percent. Intercompany transactions between CL&P, NSTAR Electric, PSNH and the CYAPC and YAEC companies have been eliminated in consolidation of the Eversource financial statements. Eversource's utility subsidiaries' electric, natural gas and water distribution and transmission businesses are subject to rate-regulation that is based on cost recovery and meets the criteria for application of accounting guidance for entities with rate-regulated operations, which considers the effect of regulation on the differences in the timing of the recognition of certain revenues and expenses from those of other businesses and industries. See Note 2, "Regulatory Accounting," for further information. Certain reclassifications of prior period data were made in the accompanying financial statements to conform to the current period presentation.
Accounting Standards	Accounting Standards Accounting Standards Issued but Not Yet Effective: In June 2016, the FASB issued Accounting Standards Update (“ASU”) 2016-13, Financial Instruments - Credit Losses (Topic 326), which provides a new model for recognizing credit losses on financial instruments based on an estimate of current expected credit losses. Under the new guidance, immediate recognition of all credit losses expected over the life of a financial instrument is required. The new standard also revises the other-than-temporary impairment model for available-for-sale debt securities. The standard is effective January 1, 2020, and requires a modified retrospective transition approach through a cumulative-effect adjustment to retained earnings. The Company is assessing the impacts of this standard on the accounting for credit losses on its financial instruments, including accounts receivable. Accounting Standards Recently Adopted: On January 1, 2019, the Company adopted ASU 2016-02, Leases (Topic 842), which amended existing lease accounting guidance. The Company applied the Topic 842 lease criteria to new leases and lease renewals entered into effective on or after January 1, 2019. The ASU required balance sheet recognition of leases deemed to be operating leases as well as additional disclosure requirements. The recognition, measurement and presentation of expenses and cash flows were not significantly changed. The Company also adopted the modified retrospective transition method allowed in ASU 2018-11, Leases (Topic 842) - Targeted Improvements, which allowed the Company to adopt the new leases standard as of January 1, 2019, with prior periods presented in the financial statements continuing to follow existing lease accounting guidance under Topic 840 (Leases) in the accounting literature. Implementation of ASU 2018-11 had no effect on retained earnings, and the requirements of the new lease standard (Topic 842) are reflected in the 2019 financial statements and footnotes. The Company elected the practical expedient package whereby it did not need to reassess whether or not an existing contract is or contains a lease or whether a lease is an operating or capital lease, and it did not need to reassess initial direct costs for leases. Election of this practical expedient allowed us to carry forward our historical lease classifications. The Company elected the practical expedient to not reevaluate land easements existing at adoption if they were not previously accounted for as leases. The Company also elected to use the discount rate as of the January 1, 2019 implementation date to discount its operating lease liabilities. The Company did not elect the hindsight practical expedient to determine the lease term for existing leases. The Company determined the impact the ASUs had on its financial statements by reviewing its lease population and identifying lease data needed for the disclosure requirements. The Company implemented a new lease accounting system in 2019 to ensure ongoing compliance with the ASU’s requirements. Adoption of the new standard resulted in the recording of operating lease liabilities and right-of-use assets on the balance sheet upon transition at January 1, 2019 of $58.0 million at Eversource, $25.3 million at NSTAR Electric, $0.6 million at CL&P, and $0.6 million at PSNH. Implementation of the new guidance did not have an impact on each company’s results of operations or cash flows.
Provision for Uncollectible Accounts	Provision for Uncollectible Accounts Eversource, including CL&P, NSTAR Electric and PSNH, presents its receivables at estimated net realizable value by maintaining a provision for uncollectible accounts. This provision is determined based upon a variety of judgments and factors, including the application of an estimated uncollectible percentage to each receivable aging category. The estimate is based upon historical collection and write-off experience and management's assessment of collectability from customers. Management continuously assesses the collectability of receivables and adjusts collectability estimates based on actual experience. Receivable balances are written off against the provision for uncollectible accounts when the customer accounts are terminated and these balances are deemed to be uncollectible. The PURA allows CL&P and Yankee Gas to accelerate the recovery of accounts receivable balances attributable to qualified customers under financial or medical duress (uncollectible hardship accounts receivable) outstanding for greater than 180 days and 90 days, respectively. The DPU allows NSTAR Electric and NSTAR Gas to recover in rates amounts associated with certain uncollectible hardship accounts receivable. These uncollectible hardship customer account balances are included in Regulatory Assets or Other Long-Term Assets on the balance sheets.
Fair Value Measurements	Fair Value Measurements Fair value measurement guidance is applied to derivative contracts that are not elected or designated as "normal purchases" or "normal sales" ("normal") and to the marketable securities held in trusts. Fair value measurement guidance is also applied to valuations of the investments used to calculate the funded status of pension and PBOP plans, the nonrecurring fair value measurements of nonfinancial assets such as goodwill and AROs, and the estimated fair value of preferred stock, long-term debt and RRBs. Fair Value Hierarchy: In measuring fair value, Eversource uses observable market data when available in order to minimize the use of unobservable inputs. Inputs used in fair value measurements are categorized into three fair value hierarchy levels for disclosure purposes. The entire fair value measurement is categorized based on the lowest level of input that is significant to the fair value measurement. Eversource evaluates the classification of assets and liabilities measured at fair value on a quarterly basis, and Eversource's policy is to recognize transfers between levels of the fair value hierarchy as of the end of the reporting period. The three levels of the fair value hierarchy are described below: Level 1 - Inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities as of the reporting date. Active markets are those in which transactions for the asset or liability occur in sufficient frequency and volume to provide pricing information on an ongoing basis. Level 2 - Inputs are quoted prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets that are not active, and model-derived valuations in which all significant inputs are observable. Level 3 - Quoted market prices are not available. Fair value is derived from valuation techniques in which one or more significant inputs or assumptions are unobservable. Where possible, valuation techniques incorporate observable market inputs that can be validated to external sources such as industry exchanges, including prices of energy and energy-related products. Uncategorized - Investments that are measured at net asset value are not categorized within the fair value hierarchy.
Other Taxes	As agents for state and local governments, Eversource's companies that serve customers in Connecticut and Massachusetts collect certain sales taxes that are recorded on a net basis with no impact on the statements of income. Separate from above are amounts recorded as Taxes Other than Income Taxes related to the future remittance to the State of Connecticut of energy efficiency funds collected from customers in Operating Revenues. These amounts are $10.7 million and $21.4 million for the three and six months ended June 30, 2019, respectively, and $12.7 million and $25.4 million for the three and six months ended June 30, 2018, respectively. These amounts are recorded separately, with collections in Operating Revenues and with payments in Taxes Other than Income Taxes on the Eversource and CL&P statements of income. Other TaxesEversource's companies that serve customers in Connecticut collect gross receipts taxes levied by the state of Connecticut from their customers.
Restricted Cash	Restricted cash included in Prepayments and Other Current Assets primarily represents cash collections related to the PSNH RRB customer charges that are held in trust, and required ISO-NE cash deposits. Restricted cash included in Marketable Securities represents money market funds held in trusts to fund certain non-qualified executive benefits and restricted trusts to fund CYAPC and YAEC's spent nuclear fuel storage facilities obligations.
Earnings Per Share	Basic EPS is computed based upon the weighted average number of common shares outstanding during each period. Diluted EPS is computed on the basis of the weighted average number of common shares outstanding plus the potential dilutive effect of certain share-based compensation awards and the equity forward sale agreement, as if they were converted into outstanding common shares. The dilutive effect of unvested RSU and performance share awards, as well as the equity forward sale agreement, is calculated using the treasury stock method. RSU and performance share awards are included in basic weighted average common shares outstanding as of the date that all necessary vesting conditions have been satisfied. As described in Note 13, "Common Shares," earnings per share dilution, if any, related to the forward sale agreement will be determined under the treasury stock method until settlement of the forward sale agreement. Under this method, the number of Eversource common shares used in calculating diluted EPS is deemed to be increased by the excess, if any, of the number of shares that would be issued upon physical settlement of the forward sale agreement less the number of shares that would be purchased by Eversource in the market (based on the average market price during the same reporting period) using the proceeds receivable upon settlement (based on the adjusted forward sale price at the end of that reporting period). Share dilution occurs when the average market price of Eversource's common shares is higher than the adjusted forward sale price.

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)

6 Months Ended

Jun. 30, 2019

Eversource Energy is a public utility holding company primarily engaged, through its wholly-owned regulated utility subsidiaries, in the energy delivery business. Eversource Energy's wholly-owned regulated utility subsidiaries consist of CL&P, NSTAR Electric and PSNH (electric utilities), Yankee Gas and NSTAR Gas (natural gas utilities) and Aquarion (water utilities). Eversource provides energy delivery and/or water service to approximately four million electric, natural gas and water customers through eight regulated utilities in Connecticut, Massachusetts and New Hampshire.

The unaudited condensed consolidated financial statements of Eversource, NSTAR Electric and PSNH include the accounts of each of their respective subsidiaries. Intercompany transactions have been eliminated in consolidation. The accompanying unaudited condensed consolidated financial statements of Eversource, NSTAR Electric and PSNH and the unaudited condensed financial statements of CL&P are herein collectively referred to as the "financial statements."

The combined notes to the financial statements have been prepared pursuant to the rules and regulations of the SEC. Certain information and footnote disclosures included in annual financial statements prepared in accordance with GAAP have been omitted pursuant to such rules and regulations. The accompanying financial statements should be read in conjunction with the Combined Notes to Financial Statements included in Item 8, "Financial Statements and Supplementary Data," of the Eversource 2018 Form 10-K, which was filed with the SEC on February 26, 2019. The preparation of the financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.

The financial statements contain, in the opinion of management, all adjustments (including normal, recurring adjustments) necessary to present fairly Eversource's, CL&P's, NSTAR Electric's and PSNH's financial position as of June 30, 2019 and December 31, 2018, and the results of operations, comprehensive income and common shareholders' equity for the three and six months ended June 30, 2019 and 2018, and the cash flows for the six months ended June 30, 2019 and 2018. The results of operations and comprehensive income for the three and six months ended June 30, 2019 and 2018 and the cash flows for the six months ended June 30, 2019 and 2018 are not necessarily indicative of the results expected for a full year.

Eversource consolidates CYAPC and YAEC because CL&P's, NSTAR Electric's and PSNH's combined ownership and voting interests in each of these entities is greater than 50 percent. Intercompany transactions between CL&P, NSTAR Electric, PSNH and the CYAPC and YAEC companies have been eliminated in consolidation of the Eversource financial statements.

Eversource's utility subsidiaries' electric, natural gas and water distribution and transmission businesses are subject to rate-regulation that is based on cost recovery and meets the criteria for application of accounting guidance for entities with rate-regulated operations, which considers the effect of regulation on the differences in the timing of the recognition of certain revenues and expenses from those of other businesses and industries. See Note 2, "Regulatory Accounting," for further information.

Certain reclassifications of prior period data were made in the accompanying financial statements to conform to the current period presentation.

Accounting Standards

Accounting Standards Issued but Not Yet Effective: In June 2016, the FASB issued Accounting Standards Update (“ASU”) 2016-13, Financial Instruments - Credit Losses (Topic 326), which provides a new model for recognizing credit losses on financial instruments based on an estimate of current expected credit losses. Under the new guidance, immediate recognition of all credit losses expected over the life of a financial instrument is required. The new standard also revises the other-than-temporary impairment model for available-for-sale debt securities. The standard is effective January 1, 2020, and requires a modified retrospective transition approach through a cumulative-effect adjustment to retained earnings. The Company is assessing the impacts of this standard on the accounting for credit losses on its financial instruments, including accounts receivable.

Accounting Standards Recently Adopted: On January 1, 2019, the Company adopted ASU 2016-02, Leases (Topic 842), which amended existing lease accounting guidance. The Company applied the Topic 842 lease criteria to new leases and lease renewals entered into effective on or after January 1, 2019. The ASU required balance sheet recognition of leases deemed to be operating leases as well as additional disclosure requirements. The recognition, measurement and presentation of expenses and cash flows were not significantly changed.

The Company also adopted the modified retrospective transition method allowed in ASU 2018-11, Leases (Topic 842) - Targeted Improvements, which allowed the Company to adopt the new leases standard as of January 1, 2019, with prior periods presented in the financial statements continuing to follow existing lease accounting guidance under Topic 840 (Leases) in the accounting literature. Implementation of ASU 2018-11 had no effect on retained earnings, and the requirements of the new lease standard (Topic 842) are reflected in the 2019 financial statements and footnotes.

The Company elected the practical expedient package whereby it did not need to reassess whether or not an existing contract is or contains a lease or whether a lease is an operating or capital lease, and it did not need to reassess initial direct costs for leases. Election of this practical expedient allowed us to carry forward our historical lease classifications. The Company elected the practical expedient to not reevaluate land easements existing at adoption if they were not previously accounted for as leases. The Company also elected to use the discount rate as of the January 1, 2019 implementation date to discount its operating lease liabilities. The Company did not elect the hindsight practical expedient to determine the lease term for existing leases.

The Company determined the impact the ASUs had on its financial statements by reviewing its lease population and identifying lease data needed for the disclosure requirements. The Company implemented a new lease accounting system in 2019 to ensure ongoing compliance with the ASU’s requirements. Adoption of the new standard resulted in the recording of operating lease liabilities and right-of-use assets on the balance sheet upon transition at January 1, 2019 of $58.0 million at Eversource, $25.3 million at NSTAR Electric, $0.6 million at CL&P, and $0.6 million at PSNH. Implementation of the new guidance did not have an impact on each company’s results of operations or cash flows.

Provision for Uncollectible Accounts

Eversource, including CL&P, NSTAR Electric and PSNH, presents its receivables at estimated net realizable value by maintaining a provision for uncollectible accounts. This provision is determined based upon a variety of judgments and factors, including the application of an estimated uncollectible percentage to each receivable aging category. The estimate is based upon historical collection and write-off experience and management's assessment of collectability from customers. Management continuously assesses the collectability of receivables and adjusts collectability estimates based on actual experience. Receivable balances are written off against the provision for uncollectible accounts when the customer accounts are terminated and these balances are deemed to be uncollectible.

The PURA allows CL&P and Yankee Gas to accelerate the recovery of accounts receivable balances attributable to qualified customers under financial or medical duress (uncollectible hardship accounts receivable) outstanding for greater than 180 days and 90 days, respectively. The DPU allows NSTAR Electric and NSTAR Gas to recover in rates amounts associated with certain uncollectible hardship accounts receivable. These uncollectible hardship customer account balances are included in Regulatory Assets or Other Long-Term Assets on the balance sheets.

Fair Value Measurements

Fair value measurement guidance is applied to derivative contracts that are not elected or designated as "normal purchases" or "normal sales" ("normal") and to the marketable securities held in trusts. Fair value measurement guidance is also applied to valuations of the investments used to calculate the funded status of pension and PBOP plans, the nonrecurring fair value measurements of nonfinancial assets such as goodwill and AROs, and the estimated fair value of preferred stock, long-term debt and RRBs.

Fair Value Hierarchy: In measuring fair value, Eversource uses observable market data when available in order to minimize the use of unobservable inputs. Inputs used in fair value measurements are categorized into three fair value hierarchy levels for disclosure purposes. The entire fair value measurement is categorized based on the lowest level of input that is significant to the fair value measurement. Eversource evaluates the classification of assets and liabilities measured at fair value on a quarterly basis, and Eversource's policy is to recognize transfers between levels of the fair value hierarchy as of the end of the reporting period. The three levels of the fair value hierarchy are described below:

Level 1 - Inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities as of the reporting date. Active markets are those in which transactions for the asset or liability occur in sufficient frequency and volume to provide pricing information on an ongoing basis.

Level 2 - Inputs are quoted prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets that are not active, and model-derived valuations in which all significant inputs are observable.

Level 3 - Quoted market prices are not available. Fair value is derived from valuation techniques in which one or more significant inputs or assumptions are unobservable. Where possible, valuation techniques incorporate observable market inputs that can be validated to external sources such as industry exchanges, including prices of energy and energy-related products.

Uncategorized - Investments that are measured at net asset value are not categorized within the fair value hierarchy.

Other Taxes

As agents for state and local governments, Eversource's companies that serve customers in Connecticut and Massachusetts collect certain sales taxes that are recorded on a net basis with no impact on the statements of income.

Separate from above are amounts recorded as Taxes Other than Income Taxes related to the future remittance to the State of Connecticut of energy efficiency funds collected from customers in Operating Revenues. These amounts are $10.7 million and $21.4 million for the three and six months ended June 30, 2019, respectively, and $12.7 million and $25.4 million for the three and six months ended June 30, 2018, respectively. These amounts are recorded separately, with collections in Operating Revenues and with payments in Taxes Other than Income Taxes on the Eversource and CL&P statements of income.

Other TaxesEversource's companies that serve customers in Connecticut collect gross receipts taxes levied by the state of Connecticut from their customers.

Restricted Cash

Restricted cash included in Prepayments and Other Current Assets primarily represents cash collections related to the PSNH RRB customer charges that are held in trust, and required ISO-NE cash deposits. Restricted cash included in Marketable Securities represents money market funds held in trusts to fund certain non-qualified executive benefits and restricted trusts to fund CYAPC and YAEC's spent nuclear fuel storage facilities obligations.

Earnings Per Share

Basic EPS is computed based upon the weighted average number of common shares outstanding during each period. Diluted EPS is computed on the basis of the weighted average number of common shares outstanding plus the potential dilutive effect of certain share-based compensation awards and the equity forward sale agreement, as if they were converted into outstanding common shares. The dilutive effect of unvested RSU and performance share awards, as well as the equity forward sale agreement, is calculated using the treasury stock method. RSU and performance share awards are included in basic weighted average common shares outstanding as of the date that all necessary vesting conditions have been satisfied.

As described in Note 13, "Common Shares," earnings per share dilution, if any, related to the forward sale agreement will be determined under the treasury stock method until settlement of the forward sale agreement. Under this method, the number of Eversource common shares used in calculating diluted EPS is deemed to be increased by the excess, if any, of the number of shares that would be issued upon physical settlement of the forward sale agreement less the number of shares that would be purchased by Eversource in the market (based on the average market price during the same reporting period) using the proceeds receivable upon settlement (based on the adjusted forward sale price at the end of that reporting period). Share dilution occurs when the average market price of Eversource's common shares is higher than the adjusted forward sale price.