REGULATORY ACCOUNTING (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014
Regulatory Asset [Line Items]
Regulatory Assets, Total	$ 4,583,800	$ 4,726,600
Regulatory Assets Current	845,843	672,493
Regulatory Assets Long Term	3,737,960	4,054,086
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	$ 75,300	60,500
Description of Regulatory Assets and Liabilities	Benefit Costs: Eversource's Pension, SERP and PBOP Plans are accounted for in accordance with accounting guidance on defined benefit pension and other PBOP plans. The liability recorded by the Regulated companies to recognize the funded status of their retiree benefit plans is offset by a regulatory asset in lieu of a charge to Accumulated Other Comprehensive Income/(Loss), reflecting ultimate recovery from customers through rates. The regulatory asset is amortized as the actuarial gains and losses and prior service cost are amortized to net periodic benefit cost for the pension and PBOP plans. All amounts are remeasured annually. Regulatory accounting is also applied to the portions of Eversource's service company costs that support the Regulated companies, as these amounts are also recoverable. As these regulatory assets do not represent a cash outlay for the Regulated companies, no carrying charge is recovered from customers. CL&P, NSTAR Electric, PSNH and WMECO recover benefit costs related to their distribution and transmission operations from customers in rates as allowed by their applicable regulatory commissions. NSTAR Electric and WMECO each recover their qualified pension and PBOP expenses related to distribution operations through rate reconciling mechanisms that fully track the change in net pension and PBOP expenses each year. Derivative Liabilities: Regulatory assets are recorded as an offset to derivative liabilities and relate to the fair value of contracts used to purchase energy and energy-related products that will be recovered from customers in future rates. These assets are excluded from rate base and are being recovered as the actual settlements occur over the duration of the contracts. See Note 4, "Derivative Instruments," to the financial statements for further information on these contracts. Income Taxes, Net: The tax effect of temporary book-tax differences (differences between the periods in which transactions affect income in the financial statements and the periods in which they affect the determination of taxable income, including those differences relating to uncertain tax positions) is accounted for in accordance with the rate-making treatment of the applicable regulatory commissions and accounting guidance for income taxes. Differences in income taxes between the accounting guidance and the rate-making treatment of the applicable regulatory commissions are recorded as regulatory assets. As these assets are offset by deferred income tax liabilities, no carrying charge is collected. The amortization period of these assets varies depending on the nature and/or remaining life of the underlying assets and liabilities. For further information regarding income taxes, see Note 10, "Income Taxes," to the financial statements. Storm Restoration Costs: The storm restoration cost deferrals relate to costs incurred for major storm events at CL&P, NSTAR Electric, PSNH and WMECO that each company expects to recover from customers. A storm must meet certain criteria to qualify as a major storm with the criteria specific to each state jurisdiction and utility company. Once a storm qualifies as a major storm, all qualifying expenses incurred during storm restoration efforts are deferred and recovered from customers. In addition to storm restoration costs, CL&P and PSNH are each allowed to recover pre-staging storm costs. Of the total deferred storm restoration costs, $197 million is pending regulatory approval (including $106 million at NSTAR Electric, $61 million at PSNH, and $30 million at WMECO). Management believes the storm restoration costs were prudent and meet the criteria for specific cost recovery in Connecticut, Massachusetts and New Hampshire, and that recovery from customers is probable through the applicable regulatory recovery process. Each electric utility has sought, or is seeking, recovery of its deferred storm restoration costs through its applicable regulatory recovery process. Each electric utility company earns a return on its deferred storm restoration cost regulatory asset balance. Goodwill-related: The goodwill regulatory asset originated from a 1999 merger transaction and the DPU allowed its recovery in NSTAR Electric and NSTAR Gas rates. This regulatory asset is currently being amortized and recovered from customers in rates without a carrying charge over a 40-year period, and, as of December 31, 2015, there were 24 years of amortization remaining. Regulatory Tracker Mechanisms: The Regulated companies’ approved rates are designed to recover their costs incurred to provide service to customers. The Regulated companies recover certain of their costs on a fully-reconciling basis through regulatory commission-approved tracking mechanisms. The differences between the costs incurred (or the rate recovery allowed) and the actual revenues are recorded as regulatory assets (for undercollections) or as regulatory liabilities (for overcollections) to be included in future customer rates each year. Carrying charges are recorded on all material regulatory tracker mechanisms. CL&P, NSTAR Electric, PSNH and WMECO each recover, on a fully reconciling basis, the costs associated with the procurement of energy, transmission related costs from FERC-approved transmission tariffs, energy efficiency programs (including LBR at NSTAR Electric), low income assistance programs, certain uncollectible accounts receivable for hardship customers, and restructuring and stranded costs as a result of deregulation. Energy procurement costs at PSNH include the costs related to its generating stations and at WMECO include the costs related to its solar generation. CL&P (effective December 1, 2014) and WMECO each have a regulatory commission approved revenue decoupling mechanism. Distribution revenues are decoupled from customer sales volumes, which breaks the relationship between sales volumes and revenues recognized. CL&P and WMECO reconcile their annual base distribution rate recovery to pre-established levels of baseline distribution delivery service revenues. Any difference between the allowed level of distribution revenue and the actual amount received during a 12-month period is adjusted through rates in the following period. CL&P and WMECO's revenue decoupling mechanisms permit recovery of an annual base amount of distribution revenues of $1.059 billion and $132.4 million, respectively. Contractual Obligations - Yankee Companies: CL&P, NSTAR Electric, PSNH and WMECO are responsible for their proportionate share of the remaining costs of the CYAPC, YAEC and MYAPC nuclear facilities, including nuclear fuel storage. A portion of these costs was recorded as a regulatory asset. Amounts for CL&P are earning a return and are being recovered through the CTA. Amounts for NSTAR Electric and WMECO are being recovered without a return through the transition charge. Amounts for PSNH were fully recovered in 2006. As a result of Eversource's consolidation of CYAPC and YAEC, Eversource's regulatory asset balance also includes the regulatory assets of CYAPC and YAEC, which totaled $110.9 million and $97.8 million as of December 31, 2015 and 2014, respectively. Intercompany transactions between CL&P, NSTAR Electric, PSNH and WMECO and the CYAPC and YAEC companies have been eliminated in consolidation of the Eversource financial statements. Other Regulatory Assets: Other Regulatory Assets primarily include asset retirement obligations, environmental remediation costs, losses associated with the reacquisition or redemption of long-term debt, purchase power contract termination costs and various other items. Regulatory Costs in Other Long-Term Assets: The Regulated companies had $75.3 million (including $3.1 million for CL&P, $35.4 million for NSTAR Electric, $4.8 million for PSNH and $16.7 million for WMECO) and $60.5 million (including $1.3 million for CL&P, $33.2 million for NSTAR Electric, $0.9 million for PSNH, and $11 million for WMECO) of additional regulatory costs as of December 31, 2015 and 2014, respectively, that were included in Other Long-Term Assets on the balance sheets. These amounts represent incurred costs for which recovery has not yet been specifically approved by the applicable regulatory agency. However, based on regulatory policies or past precedent on similar costs, management believes it is probable that these costs will ultimately be approved and recovered from customers in rates. The NSTAR Electric balance as of December 31, 2015 and 2014 primarily related to the deferral of certain bad debt costs expected to be recovered in future rates. Equity Return on Regulatory Assets: For rate-making purposes, the Regulated companies recover the carrying costs related to their regulatory assets. For certain regulatory assets, the carrying cost recovered includes an equity return component. This equity return, which is not recorded on the balance sheets, totaled $1.5 million and $1.7 million for CL&P as of December 31, 2015 and 2014, respectively. These carrying costs will be recovered from customers in future rates. As of December 31, 2015 and 2014, this equity return, which is not recorded on the balance sheets, totaled $48.3 million and $43.3 million, respectively, for PSNH. These amounts include $25 million of equity return on the Clean Air Project costs that PSNH has agreed not to bill customers pending NHPUC approval of a generation divestiture settlement agreement. For further information on the divestiture, see Note 11H, "Commitments and Contingencies – PSNH Generation Restructuring." Regulatory Liabilities: The components of regulatory liabilities were as follows: Cost of Removal: Eversource's Regulated companies currently recover amounts in rates for future costs of removal of plant assets over the lives of the assets. The estimated cost to remove utility assets from service is recognized as a component of depreciation expense and the cumulative amount collected from customers but not yet expended is recognized as a regulatory liability. Expended costs that exceed amounts collected from customers are recognized as regulatory assets, as they are probable of recovery in future rates. AFUDC - Transmission: Regulatory liabilities were recorded by CL&P and WMECO for AFUDC accrued on certain reliability-related transmission projects to reflect local rate base recovery as a result of a FERC-approved transmission tariff. A regulatory liability was recorded by NSTAR Electric for AFUDC accrued on certain reliability-related transmission projects through December 31, 2015 to reflect local rate base recovery. These regulatory liabilities for CL&P, NSTAR Electric and WMECO will be amortized over the depreciable life of the related transmission assets.
Public Utilities Disclosure Of Regulatory Matters	2015 Regulatory Developments: FERC ROE Complaints: As a result of the actions taken by the FERC and other developments in the pending ROE complaint proceedings described in Note 11E, "Commitments and Contingencies – FERC ROE Complaints," Eversource recorded reserves for the first and second ROE complaints, which were recorded as a regulatory liability and as a reduction to operating revenues. The cumulative pre-tax reserves (excluding interest) as of December 31, 2015, which include the impact of refunds given to customers, totaled $39.1 million for Eversource (including $21.4 million for CL&P, $8.5 million for NSTAR Electric, $3.1 million for PSNH, and $6.1 million for WMECO). NSTAR Electric and NSTAR Gas Comprehensive Settlement Agreement: On March 2, 2015, the DPU approved the comprehensive settlement agreement between NSTAR Electric, NSTAR Gas and the Massachusetts Attorney General (the "Settlement") as filed with the DPU on December 31, 2014. The Settlement resolved the outstanding NSTAR Electric CPSL program filings for 2006 through 2011, the NSTAR Electric and NSTAR Gas PAM and energy efficiency-related customer billing adjustments reported in 2012, and the recovery of LBR related to NSTAR Electric's energy efficiency programs for 2009 through 2011 (11 dockets in total). In 2015, as a result of the DPU order, NSTAR Electric and NSTAR Gas commenced refunding a combined $44.7 million to customers, which was recorded as a regulatory liability. Refunds to customers will continue through December 2016. As a result of the Settlement, NSTAR Electric increased its operating revenues and decreased its amortization expense in 2015, resulting in the recognition of a $21.7 million pre-tax benefit in 2015. NSTAR Electric Basic Service Bad Debt Adder: On January 7, 2015, the DPU issued an order concluding that NSTAR Electric had removed energy-related bad debt costs from base distribution rates effective January 1, 2006. As a result of the DPU order, in the first quarter of 2015, NSTAR Electric increased its regulatory assets and reduced its operations and maintenance expense by an under recovered amount of $24.2 million for energy-related bad debt costs through 2014, resulting in a pre-tax benefit in 2015. NSTAR Electric filed for recovery of the energy-related bad debt costs regulatory asset from customers and on November 20, 2015 the DPU approved NSTAR Electric’s proposed rate increase to recover these costs over a 12-month period, effective January 1, 2016. CL&P Distribution Rates: On July 2, 2015, PURA issued a final order that approved a settlement agreement filed on May 19, 2015, which allows for an increase to rate base of approximately $163 million associated with ADIT, including a regulatory asset to recover the incremental revenue requirement for the period December 1, 2014 through November 30, 2015 over a subsequent 24-month period. The rate base increase provided an increase to total allowed annual revenue requirements of $18.4 million beginning December 1, 2014. As part of the settlement agreement, the $18.4 million for the period December 1, 2014 through November 30, 2015 was recorded as a regulatory asset with a corresponding increase in Operating Revenues, and is being collected from customers in rates over a 24-month period beginning December 1, 2015. NSTAR Gas Distribution Rates: On October 30, 2015, the DPU issued its order in the NSTAR Gas distribution rate case, which approved an annualized base rate increase of $15.8 million, plus other increases of approximately $11.5 million, mostly relating to recovery of pension and PBOP expenses and the Hopkinton GSA, effective January 1, 2016. In the order, the DPU also approved an authorized regulatory ROE of 9.8 percent, the establishment of a revenue decoupling mechanism, the recovery of certain bad debt expenses, and a 52.1 percent equity component of its capital structure. On November 19, 2015, NSTAR Gas filed a motion for reconsideration of the order with the DPU seeking the correction of mathematical errors and other plant and cost of service items. As a result of this order, Eversource recorded regulatory deferrals for costs that have been approved for recovery or are expected to be approved for recovery in future rate proceedings, which resulted in the recognition of a $17.2 million pre-tax benefit in 2015. Included in this amount is a $10.5 million pre-tax benefit recorded at NSTAR Electric for certain uncollectible hardship accounts receivable that are expected to be recovered in future rates given the allowed recoveries of uncollectible hardship accounts receivable by WMECO and NSTAR Gas.
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	$ 621,400	750,100
Regulatory Liability Current	107,759	235,022
Regulatory Liabilities Long-Term	513,595	515,144
The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	1,637,300	1,695,800
Regulatory Assets Current	268,318	220,344
Regulatory Assets Long Term	1,369,028	1,475,508
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	3,100	1,300
Public Utilities Amount Of Allowance For Earnings On Equity Capitalized For Rate Making Purposes	1,500	1,700
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	136,000	206,000
Regulatory Liability Current	61,155	124,722
Regulatory Liabilities Long-Term	74,830	81,298
NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	1,461,400	1,377,800
Regulatory Assets Current	348,408	198,710
Regulatory Assets Long Term	1,112,977	1,179,100
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	35,400	33,200
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	267,700	312,300
Regulatory Liability Current	3,281	49,611
Regulatory Liabilities Long-Term	264,352	262,738
Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	362,900	404,800
Regulatory Assets Current	104,971	111,705
Regulatory Assets Long Term	257,873	293,115
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	4,800	900
Public Utilities Amount Of Allowance For Earnings On Equity Capitalized For Rate Making Purposes	48,300	43,300
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	54,800	67,400
Regulatory Liability Current	6,898	16,044
Regulatory Liabilities Long-Term	47,851	51,372
Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	191,200	198,200
Regulatory Assets Current	56,166	51,923
Regulatory Assets Long Term	135,010	146,307
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	16,700	11,000
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	24,700	33,300
Regulatory Liability Current	13,122	22,486
Regulatory Liabilities Long-Term	11,597	10,835
CYAPC and YAEC [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	110,900	97,800
Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	437,100	439,900
Cost Of Removal [Member] \| The Connecticut Light And Power Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	24,100	19,700
Cost Of Removal [Member] \| NSTAR Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	257,400	258,300
Cost Of Removal [Member] \| Public Service Company Of New Hampshire [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	47,200	50,300
Cost Of Removal [Member] \| Western Massachusetts Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	2,800	1,100
Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	99,700	192,300
Regulatory Tracker Deferrals Regulatory Liabilities [Member] \| The Connecticut Light And Power Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	56,200	122,600
Regulatory Tracker Deferrals Regulatory Liabilities [Member] \| NSTAR Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	3,300	20,700
Regulatory Tracker Deferrals Regulatory Liabilities [Member] \| Public Service Company Of New Hampshire [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	3,400	14,200
Regulatory Tracker Deferrals Regulatory Liabilities [Member] \| Western Massachusetts Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	12,900	22,300
AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	66,100	67,100
AFUDC Transmission Incentive [Member] \| The Connecticut Light And Power Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	51,500	53,600
AFUDC Transmission Incentive [Member] \| NSTAR Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	5,700	4,400
AFUDC Transmission Incentive [Member] \| Public Service Company Of New Hampshire [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	0	0
AFUDC Transmission Incentive [Member] \| Western Massachusetts Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	8,900	9,100
Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	18,500	50,800
Other Regulatory Liabilities [Member] \| The Connecticut Light And Power Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	4,200	10,100
Other Regulatory Liabilities [Member] \| NSTAR Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	1,300	28,900
Other Regulatory Liabilities [Member] \| Public Service Company Of New Hampshire [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	4,200	2,900
Other Regulatory Liabilities [Member] \| Western Massachusetts Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	100	800
Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	1,828,200	2,016,000
Benefit Costs [Member] \| The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	413,600	445,400
Benefit Costs [Member] \| NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	479,900	515,900
Benefit Costs [Member] \| Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	164,200	174,300
Benefit Costs [Member] \| Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	84,900	85,000
Regulatory Assets Offsetting Derivative Liabilities [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	388,000	425,500
Regulatory Assets Offsetting Derivative Liabilities [Member] \| The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	380,800	410,900
Regulatory Assets Offsetting Derivative Liabilities [Member] \| NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	1,300	4,500
Regulatory Assets Offsetting Derivative Liabilities [Member] \| Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	0	0
Regulatory Assets Offsetting Derivative Liabilities [Member] \| Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	0	0
Deferred Income Tax Charges Member
Regulatory Asset [Line Items]
Regulatory Assets, Total	650,900	635,300
Deferred Income Tax Charges Member \| The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	444,400	437,700
Deferred Income Tax Charges Member \| NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	85,700	83,700
Deferred Income Tax Charges Member \| Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	34,500	38,000
Deferred Income Tax Charges Member \| Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	31,800	35,500
Storm Restoration Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	436,900	502,800
Storm Restoration Costs [Member] \| The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	271,400	319,600
Storm Restoration Costs [Member] \| NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	110,900	103,700
Storm Restoration Costs [Member] \| Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	31,500	47,700
Storm Restoration Costs [Member] \| Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	23,100	31,800
Goodwill Regulatory Asset [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	484,900	505,400
Goodwill Regulatory Asset [Member] \| NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	416,300	433,900
Regulatory Tracker Deferrals Regulatory Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	526,500	350,500
Regulatory Tracker Deferrals Regulatory Assets [Member] \| The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	45,100	16,100
Regulatory Tracker Deferrals Regulatory Assets [Member] \| NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	311,000	141,400
Regulatory Tracker Deferrals Regulatory Assets [Member] \| Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	101,200	103,500
Regulatory Tracker Deferrals Regulatory Assets [Member] \| Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	40,100	33,000
Unrecovered Contractual Obligations [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	134,400	123,800
Other Regulatory Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	134,000	167,300
Other Regulatory Assets [Member] \| The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	82,000	66,100
Other Regulatory Assets [Member] \| NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	56,300	94,700
Other Regulatory Assets [Member] \| Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	31,500	41,300
Other Regulatory Assets [Member] \| Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	$ 11,300	$ 12,900

REGULATORY ACCOUNTING (Details) - USD ($)
$ in Thousands

12 Months Ended

Dec. 31, 2015

Dec. 31, 2014

Regulatory Asset [Line Items]

Regulatory Assets, Total

$ 4,583,800

$ 4,726,600

Regulatory Assets Current

845,843

672,493

Regulatory Assets Long Term

3,737,960

4,054,086

Regulatory Assets And Liabilities Other Disclosures Abstract

Amount of Regulatory Costs Not yet Approved

$ 75,300

60,500

Description of Regulatory Assets and Liabilities

Benefit Costs: Eversource's Pension, SERP and PBOP Plans are accounted for in accordance with accounting guidance on defined benefit pension and other PBOP plans. The liability recorded by the Regulated companies to recognize the funded status of their retiree benefit plans is offset by a regulatory asset in lieu of a charge to Accumulated Other Comprehensive Income/(Loss), reflecting ultimate recovery from customers through rates. The regulatory asset is amortized as the actuarial gains and losses and prior service cost are amortized to net periodic benefit cost for the pension and PBOP plans. All amounts are remeasured annually. Regulatory accounting is also applied to the portions of Eversource's service company costs that support the Regulated companies, as these amounts are also recoverable. As these regulatory assets do not represent a cash outlay for the Regulated companies, no carrying charge is recovered from customers. CL&P, NSTAR Electric, PSNH and WMECO recover benefit costs related to their distribution and transmission operations from customers in rates as allowed by their applicable regulatory commissions. NSTAR Electric and WMECO each recover their qualified pension and PBOP expenses related to distribution operations through rate reconciling mechanisms that fully track the change in net pension and PBOP expenses each year. Derivative Liabilities: Regulatory assets are recorded as an offset to derivative liabilities and relate to the fair value of contracts used to purchase energy and energy-related products that will be recovered from customers in future rates. These assets are excluded from rate base and are being recovered as the actual settlements occur over the duration of the contracts. See Note 4, "Derivative Instruments," to the financial statements for further information on these contracts. Income Taxes, Net: The tax effect of temporary book-tax differences (differences between the periods in which transactions affect income in the financial statements and the periods in which they affect the determination of taxable income, including those differences relating to uncertain tax positions) is accounted for in accordance with the rate-making treatment of the applicable regulatory commissions and accounting guidance for income taxes. Differences in income taxes between the accounting guidance and the rate-making treatment of the applicable regulatory commissions are recorded as regulatory assets. As these assets are offset by deferred income tax liabilities, no carrying charge is collected. The amortization period of these assets varies depending on the nature and/or remaining life of the underlying assets and liabilities. For further information regarding income taxes, see Note 10, "Income Taxes," to the financial statements. Storm Restoration Costs: The storm restoration cost deferrals relate to costs incurred for major storm events at CL&P, NSTAR Electric, PSNH and WMECO that each company expects to recover from customers. A storm must meet certain criteria to qualify as a major storm with the criteria specific to each state jurisdiction and utility company. Once a storm qualifies as a major storm, all qualifying expenses incurred during storm restoration efforts are deferred and recovered from customers. In addition to storm restoration costs, CL&P and PSNH are each allowed to recover pre-staging storm costs. Of the total deferred storm restoration costs, $197 million is pending regulatory approval (including $106 million at NSTAR Electric, $61 million at PSNH, and $30 million at WMECO). Management believes the storm restoration costs were prudent and meet the criteria for specific cost recovery in Connecticut, Massachusetts and New Hampshire, and that recovery from customers is probable through the applicable regulatory recovery process. Each electric utility has sought, or is seeking, recovery of its deferred storm restoration costs through its applicable regulatory recovery process. Each electric utility company earns a return on its deferred storm restoration cost regulatory asset balance. Goodwill-related: The goodwill regulatory asset originated from a 1999 merger transaction and the DPU allowed its recovery in NSTAR Electric and NSTAR Gas rates. This regulatory asset is currently being amortized and recovered from customers in rates without a carrying charge over a 40-year period, and, as of December 31, 2015, there were 24 years of amortization remaining. Regulatory Tracker Mechanisms: The Regulated companies’ approved rates are designed to recover their costs incurred to provide service to customers. The Regulated companies recover certain of their costs on a fully-reconciling basis through regulatory commission-approved tracking mechanisms. The differences between the costs incurred (or the rate recovery allowed) and the actual revenues are recorded as regulatory assets (for undercollections) or as regulatory liabilities (for overcollections) to be included in future customer rates each year. Carrying charges are recorded on all material regulatory tracker mechanisms. CL&P, NSTAR Electric, PSNH and WMECO each recover, on a fully reconciling basis, the costs associated with the procurement of energy, transmission related costs from FERC-approved transmission tariffs, energy efficiency programs (including LBR at NSTAR Electric), low income assistance programs, certain uncollectible accounts receivable for hardship customers, and restructuring and stranded costs as a result of deregulation. Energy procurement costs at PSNH include the costs related to its generating stations and at WMECO include the costs related to its solar generation. CL&P (effective December 1, 2014) and WMECO each have a regulatory commission approved revenue decoupling mechanism. Distribution revenues are decoupled from customer sales volumes, which breaks the relationship between sales volumes and revenues recognized. CL&P and WMECO reconcile their annual base distribution rate recovery to pre-established levels of baseline distribution delivery service revenues. Any difference between the allowed level of distribution revenue and the actual amount received during a 12-month period is adjusted through rates in the following period. CL&P and WMECO's revenue decoupling mechanisms permit recovery of an annual base amount of distribution revenues of $1.059 billion and $132.4 million, respectively. Contractual Obligations - Yankee Companies: CL&P, NSTAR Electric, PSNH and WMECO are responsible for their proportionate share of the remaining costs of the CYAPC, YAEC and MYAPC nuclear facilities, including nuclear fuel storage. A portion of these costs was recorded as a regulatory asset. Amounts for CL&P are earning a return and are being recovered through the CTA. Amounts for NSTAR Electric and WMECO are being recovered without a return through the transition charge. Amounts for PSNH were fully recovered in 2006. As a result of Eversource's consolidation of CYAPC and YAEC, Eversource's regulatory asset balance also includes the regulatory assets of CYAPC and YAEC, which totaled $110.9 million and $97.8 million as of December 31, 2015 and 2014, respectively. Intercompany transactions between CL&P, NSTAR Electric, PSNH and WMECO and the CYAPC and YAEC companies have been eliminated in consolidation of the Eversource financial statements. Other Regulatory Assets: Other Regulatory Assets primarily include asset retirement obligations, environmental remediation costs, losses associated with the reacquisition or redemption of long-term debt, purchase power contract termination costs and various other items. Regulatory Costs in Other Long-Term Assets: The Regulated companies had $75.3 million (including $3.1 million for CL&P, $35.4 million for NSTAR Electric, $4.8 million for PSNH and $16.7 million for WMECO) and $60.5 million (including $1.3 million for CL&P, $33.2 million for NSTAR Electric, $0.9 million for PSNH, and $11 million for WMECO) of additional regulatory costs as of December 31, 2015 and 2014, respectively, that were included in Other Long-Term Assets on the balance sheets. These amounts represent incurred costs for which recovery has not yet been specifically approved by the applicable regulatory agency. However, based on regulatory policies or past precedent on similar costs, management believes it is probable that these costs will ultimately be approved and recovered from customers in rates. The NSTAR Electric balance as of December 31, 2015 and 2014 primarily related to the deferral of certain bad debt costs expected to be recovered in future rates. Equity Return on Regulatory Assets: For rate-making purposes, the Regulated companies recover the carrying costs related to their regulatory assets. For certain regulatory assets, the carrying cost recovered includes an equity return component. This equity return, which is not recorded on the balance sheets, totaled $1.5 million and $1.7 million for CL&P as of December 31, 2015 and 2014, respectively. These carrying costs will be recovered from customers in future rates. As of December 31, 2015 and 2014, this equity return, which is not recorded on the balance sheets, totaled $48.3 million and $43.3 million, respectively, for PSNH. These amounts include $25 million of equity return on the Clean Air Project costs that PSNH has agreed not to bill customers pending NHPUC approval of a generation divestiture settlement agreement. For further information on the divestiture, see Note 11H, "Commitments and Contingencies – PSNH Generation Restructuring." Regulatory Liabilities: The components of regulatory liabilities were as follows: Cost of Removal: Eversource's Regulated companies currently recover amounts in rates for future costs of removal of plant assets over the lives of the assets. The estimated cost to remove utility assets from service is recognized as a component of depreciation expense and the cumulative amount collected from customers but not yet expended is recognized as a regulatory liability. Expended costs that exceed amounts collected from customers are recognized as regulatory assets, as they are probable of recovery in future rates. AFUDC - Transmission: Regulatory liabilities were recorded by CL&P and WMECO for AFUDC accrued on certain reliability-related transmission projects to reflect local rate base recovery as a result of a FERC-approved transmission tariff. A regulatory liability was recorded by NSTAR Electric for AFUDC accrued on certain reliability-related transmission projects through December 31, 2015 to reflect local rate base recovery. These regulatory liabilities for CL&P, NSTAR Electric and WMECO will be amortized over the depreciable life of the related transmission assets.

Public Utilities Disclosure Of Regulatory Matters

2015 Regulatory Developments: FERC ROE Complaints: As a result of the actions taken by the FERC and other developments in the pending ROE complaint proceedings described in Note 11E, "Commitments and Contingencies – FERC ROE Complaints," Eversource recorded reserves for the first and second ROE complaints, which were recorded as a regulatory liability and as a reduction to operating revenues. The cumulative pre-tax reserves (excluding interest) as of December 31, 2015, which include the impact of refunds given to customers, totaled $39.1 million for Eversource (including $21.4 million for CL&P, $8.5 million for NSTAR Electric, $3.1 million for PSNH, and $6.1 million for WMECO). NSTAR Electric and NSTAR Gas Comprehensive Settlement Agreement: On March 2, 2015, the DPU approved the comprehensive settlement agreement between NSTAR Electric, NSTAR Gas and the Massachusetts Attorney General (the "Settlement") as filed with the DPU on December 31, 2014. The Settlement resolved the outstanding NSTAR Electric CPSL program filings for 2006 through 2011, the NSTAR Electric and NSTAR Gas PAM and energy efficiency-related customer billing adjustments reported in 2012, and the recovery of LBR related to NSTAR Electric's energy efficiency programs for 2009 through 2011 (11 dockets in total). In 2015, as a result of the DPU order, NSTAR Electric and NSTAR Gas commenced refunding a combined $44.7 million to customers, which was recorded as a regulatory liability. Refunds to customers will continue through December 2016. As a result of the Settlement, NSTAR Electric increased its operating revenues and decreased its amortization expense in 2015, resulting in the recognition of a $21.7 million pre-tax benefit in 2015. NSTAR Electric Basic Service Bad Debt Adder: On January 7, 2015, the DPU issued an order concluding that NSTAR Electric had removed energy-related bad debt costs from base distribution rates effective January 1, 2006. As a result of the DPU order, in the first quarter of 2015, NSTAR Electric increased its regulatory assets and reduced its operations and maintenance expense by an under recovered amount of $24.2 million for energy-related bad debt costs through 2014, resulting in a pre-tax benefit in 2015. NSTAR Electric filed for recovery of the energy-related bad debt costs regulatory asset from customers and on November 20, 2015 the DPU approved NSTAR Electric’s proposed rate increase to recover these costs over a 12-month period, effective January 1, 2016. CL&P Distribution Rates: On July 2, 2015, PURA issued a final order that approved a settlement agreement filed on May 19, 2015, which allows for an increase to rate base of approximately $163 million associated with ADIT, including a regulatory asset to recover the incremental revenue requirement for the period December 1, 2014 through November 30, 2015 over a subsequent 24-month period. The rate base increase provided an increase to total allowed annual revenue requirements of $18.4 million beginning December 1, 2014. As part of the settlement agreement, the $18.4 million for the period December 1, 2014 through November 30, 2015 was recorded as a regulatory asset with a corresponding increase in Operating Revenues, and is being collected from customers in rates over a 24-month period beginning December 1, 2015. NSTAR Gas Distribution Rates: On October 30, 2015, the DPU issued its order in the NSTAR Gas distribution rate case, which approved an annualized base rate increase of $15.8 million, plus other increases of approximately $11.5 million, mostly relating to recovery of pension and PBOP expenses and the Hopkinton GSA, effective January 1, 2016. In the order, the DPU also approved an authorized regulatory ROE of 9.8 percent, the establishment of a revenue decoupling mechanism, the recovery of certain bad debt expenses, and a 52.1 percent equity component of its capital structure. On November 19, 2015, NSTAR Gas filed a motion for reconsideration of the order with the DPU seeking the correction of mathematical errors and other plant and cost of service items. As a result of this order, Eversource recorded regulatory deferrals for costs that have been approved for recovery or are expected to be approved for recovery in future rate proceedings, which resulted in the recognition of a $17.2 million pre-tax benefit in 2015. Included in this amount is a $10.5 million pre-tax benefit recorded at NSTAR Electric for certain uncollectible hardship accounts receivable that are expected to be recovered in future rates given the allowed recoveries of uncollectible hardship accounts receivable by WMECO and NSTAR Gas.