SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	3 Months Ended
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	Mar. 31, 2018
Accounting Policies [Abstract]
Accounting Standards	Accounting Standards Accounting Standards Issued but Not Yet Effective: In February 2016, the FASB issued Accounting Standards Update ("ASU") 2016-02, Leases, which changes existing lease accounting guidance and is required to be applied in the first quarter of 2019, with earlier application permitted. The ASU lease criteria are required to be applied to leases and lease renewals entered into effective January 1, 2019. The Company will implement the ASU in the first quarter of 2019. The requirements of the ASU include balance sheet recognition of leases previously deemed to be operating leases, and additional disclosure requirements. The Company is in the process of evaluating what impact the ASU will have on its financial statements. As of December 31, 2017, Eversource’s total future undiscounted minimum rental payments, excluding executory costs, under long-term noncancelable operating and capital leases were less than $100 million. Accounting Standards Recently Adopted: On January 1, 2018, Eversource, CL&P, NSTAR Electric and PSNH adopted ASU 2014-09, Revenue from Contracts with Customers, which amended existing revenue recognition guidance, using the modified retrospective method (cumulatively at the date of initial application) applying it only to contracts that were not complete at January 1, 2018. Under this method of adoption, prior year reported results were not restated. Implementation of the ASU did not have a material effect on the results of operations, financial position or cash flows of Eversource, CL&P, NSTAR Electric or PSNH. See Note 15, "Revenues," for further information. The Company identified an item that was accounted for differently under the new revenue guidance, as compared to the previously existing guidance. As a result of applying guidance on the unit of account under the new standard, purchases of power from and sales of power to ISO-New England are now accounted for net by the hour, rather than net by the month. This change increased Operating Revenues and Purchased Power, Fuel and Transmission by $19 million for the three months ended March 31, 2018, with no impact on net income. On January 1, 2018, Eversource adopted ASU 2016-01, Financial Instruments - Overall: Recognition and Measurement of Financial Assets and Liabilities. The ASU removed the available-for-sale designation for equity securities, whereby changes in fair value were previously recorded in accumulated other comprehensive income within shareholders' equity, and required changes in fair value of all equity securities to be recorded in earnings effective January 1, 2018. There was no cumulative effect of adoption. For the three months ended March 31, 2018, unrealized losses recorded in Other Income, Net were $0.7 million. For further information, see Note 5, "Marketable Securities," to the financial statements. On January 1, 2018, Eversource, CL&P, NSTAR Electric and PSNH adopted ASU 2017-07, Compensation – Retirement Benefits: Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost. The ASU required separate presentation of service cost from other components of net pension and PBOP costs, with the other components presented as non-operating income and not subject to capitalization. The ASU has been applied retrospectively for the separate presentation in the income statement of service costs and other components and prospectively in the balance sheet for the capitalization of only the service cost component. As of March 31, 2018, the non-service cost components of net pension, SERP and PBOP costs that were not capitalized in plant were recorded as an increase to regulatory liabilities of approximately $10 million, as these amounts continue to be included in rates. See Note 1F, "Summary of Significant Accounting Policies - Other Income, Net," to the financial statements for the portion of pension and PBOP costs that are presented as non-operating income for the three months ended March 31, 2018 and 2017. The March 31, 2017 amounts, which were previously presented within Operations and Maintenance expense on the statements of income, totaled $8.0 million at Eversource, $0.5 million at CL&P, $5.0 million at NSTAR Electric and $1.6 million at PSNH, and have been retrospectively presented to Other Income, Net. On January 1, 2018, Eversource, CL&P, NSTAR Electric and PSNH adopted two accounting standards relating to the statement of cash flows; ASU 2016-15, Classification of Certain Cash Receipts and Cash Payments, and ASU 2016-18, Restricted Cash. As a result of implementing ASU 2016-15, dividends from equity method investments of $6.2 million and $4.8 million in the first quarter of 2018 and 2017, respectively, are presented in operating activities at Eversource, for which the 2017 amounts were previously classified in investing activities. ASU 2016-18 required that the statement of cash flows explain the change during the period in the total of cash, cash equivalents, and amounts generally described as restricted cash and restricted cash equivalents. Both standards were applied retrospectively, as required, and neither had a material impact on Eversource's, CL&P's, NSTAR Electric's or PSNH's statements of cash flows. See Note 1H, "Summary of Significant Accounting Policies - Supplemental Cash Flow Information," to the financial statements for a reconciliation of cash and cash equivalents as reported on the balance sheet to the statement of cash flows, which includes amounts described as restricted cash and restricted cash equivalents. C. Northern Pass Northern Pass is Eversource's planned 1,090 MW HVDC transmission line that will interconnect from the Québec-New Hampshire border to Franklin, New Hampshire and an associated alternating current radial transmission line between Franklin and Deerfield, New Hampshire. As of March 31, 2018, our capitalized Northern Pass project costs were approximately $289 million. In March 2018, the Massachusetts EDCs terminated contract negotiations with and conditional selection of Northern Pass under the Massachusetts Clean Energy RFP, and the New Hampshire Site Evaluation Committee ("NHSEC") issued a written decision denying Northern Pass’ siting application. NPT reviewed the written decision and filed a motion for rehearing with the NHSEC on April 27, 2018. The NHSEC denial of Northern Pass' siting application caused us to review the recoverability of our Northern Pass project costs. In this recoverability review, we estimated undiscounted expected project cash flows and compared the result to our estimated project costs to determine whether the recorded amount is recoverable. Our undiscounted cash flows were substantially in excess of our estimated project costs. We completed this analysis and concluded that our project costs are recoverable based on our expectation that the Northern Pass project remains probable of being placed in service. Consistent with Eversource’s and HQ’s long-term relationship to bring clean energy into New England, Eversource and HQ remain committed to Northern Pass and the many benefits this project will bring to our customers and the region. If as a result of future events and changes in circumstances a new recoverability review were to conclude that our project costs are not recoverable, then we would reduce Northern Pass' project costs to the estimated fair value, which could result in most of our $289 million of capitalized project costs being written off. Such a write off could have a material adverse effect on our financial position and results of operations.
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 and long-term debt. 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.
Other Taxes	Other Taxes Eversource's companies that serve customers in Connecticut collect gross receipts taxes levied by the state of Connecticut from their customers. 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.

Disclosure of accounting policy for income taxes, which may include its accounting policies for recognizing and measuring deferred tax assets and liabilities and related valuation allowances, recognizing investment tax credits, operating loss carryforwards, tax credit carryforwards, and other carryforwards, methodologies for determining its effective income tax rate and the characterization of interest and penalties in the financial statements.

- Definition

Disclosure of accounting policy for trade and other accounts receivable, and finance, loan and lease receivables, including those classified as held for investment and held for sale. This disclosure may include (1) the basis at which such receivables are carried in the entity's statements of financial position (2) how the level of the valuation allowance for receivables is determined (3) when impairments, charge-offs or recoveries are recognized for such receivables (4) the treatment of origination fees and costs, including the amortization method for net deferred fees or costs (5) the treatment of any premiums or discounts or unearned income (6) the entity's income recognition policies for such receivables, including those that are impaired, past due or placed on nonaccrual status and (7) the treatment of foreclosures or repossessions (8) the nature and amount of any guarantees to repurchase receivables.

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