REGULATORY ACCOUNTING (Details) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Regulatory Asset [Line Items]
Regulatory Assets, Total	$ 4,726,400	$ 4,726,600
Regulatory Assets Current	709,660	672,493
Regulatory Assets Long Term	4,016,684	4,054,086
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	$ 60,800	60,500
Public Utilities Disclosure Of Regulatory Matters	2015 Regulatory Developments: FERC ROE Complaints: As a result of the March 3, 2015 FERC order in the pending ROE complaint proceedings described in Note 8C, "Commitments and Contingencies – FERC ROE Complaints," in 2015, Eversource recognized a pre-tax charge to earnings (excluding interest) of $20 million, of which $12.5 million was recorded at CL&P, $2.4 million at NSTAR Electric, $1 million at PSNH, and $4.1 million at WMECO. The pre-tax charge was recorded as a regulatory liability and as a reduction to Operating Revenues. NSTAR Electric and NSTAR Gas 2014 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. NSTAR Electric recognized a $21.7 million pre-tax benefit in the first half of 2015 as a result of the approval of the Settlement. 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. The DPU ordered NSTAR Electric and the Massachusetts Attorney General to collaborate on the reconciliations of energy-related bad debt costs through 2014. As a result of the DPU order, NSTAR Electric increased its regulatory assets and reduced operations and maintenance expense by $24.2 million in the first quarter, resulting in after-tax earnings of $14.5 million. On May 5, 2015, NSTAR Electric filed for recovery of the energy-related bad debt costs regulatory asset from customers beginning July 1, 2015. On June 24, 2015, the DPU delayed the effective date of NSTAR Electric’s proposed rate increase from July 1, 2015 to November 1, 2015 to allow for the DPU staff to review the reconciliations. The established procedural schedule is expected to result in an approval of the proposed rate increase in the fourth quarter of 2015. CL&P Distribution Rates: On December 17, 2014, PURA granted a re-opener request to CL&P’s base distribution rate application for further review of the appropriate balance of ADIT utilized in the calculation of rate base. On July 2, 2015, PURA issued a final order that approved a settlement agreement filed on May 19, 2015 between CL&P and the PURA Prosecutorial Staff. The order allows for an increase to rate base of approximately $166 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 two-year period. The rate base increase provided an increase to total allowed annual revenue requirements of $18.4 million beginning December 1, 2014. Of that amount, $10.7 million has been recorded as a regulatory asset in June 2015, with a corresponding increase in Operating Revenues. The remaining $7.7 million will be recorded from July 2015 through November 2015. The aggregate amount will be collected from customers in rates over a 24-month period commencing on December 1, 2015.
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	$ 719,300	750,100
Regulatory Liability Current	208,510	235,022
Regulatory Liabilities Long-Term	510,807	515,144
The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	1,725,200	1,695,800
Regulatory Assets Current	263,713	220,344
Regulatory Assets Long Term	1,461,483	1,475,508
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	1,900	1,300
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	193,800	206,000
Regulatory Liability Current	122,617	124,722
Regulatory Liabilities Long-Term	71,245	81,298
NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	1,456,700	1,377,800
Regulatory Assets Current	258,489	198,710
Regulatory Assets Long Term	1,198,167	1,179,100
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	21,900	33,200
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	271,600	312,300
Regulatory Liability Current	5,007	49,611
Regulatory Liabilities Long-Term	266,572	262,738
Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	371,000	404,800
Regulatory Assets Current	88,773	111,705
Regulatory Assets Long Term	282,215	293,115
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	2,000	900
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	55,900	67,400
Regulatory Liability Current	5,602	16,044
Regulatory Liabilities Long-Term	50,250	51,372
Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	193,000	198,200
Regulatory Assets Current	54,926	51,923
Regulatory Assets Long Term	138,138	146,307
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	15,400	11,000
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	29,700	33,300
Regulatory Liability Current	16,847	22,486
Regulatory Liabilities Long-Term	12,921	10,835
Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	434,900	439,900
Cost Of Removal [Member] | The Connecticut Light And Power Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	13,500	19,700
Cost Of Removal [Member] | NSTAR Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	261,500	258,300
Cost Of Removal [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	49,100	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	198,000	192,300
Regulatory Tracker Deferrals Regulatory Liabilities [Member] | The Connecticut Light And Power Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	115,600	122,600
Regulatory Tracker Deferrals Regulatory Liabilities [Member] | NSTAR Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	2,900	20,700
Regulatory Tracker Deferrals Regulatory Liabilities [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	4,800	14,200
Regulatory Tracker Deferrals Regulatory Liabilities [Member] | Western Massachusetts Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	17,000	22,300
AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	66,200	67,100
AFUDC Transmission Incentive [Member] | The Connecticut Light And Power Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	52,200	53,600
AFUDC Transmission Incentive [Member] | NSTAR Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	5,000	4,400
AFUDC Transmission Incentive [Member] | Western Massachusetts Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	9,000	9,100
Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	20,200	50,800
Other Regulatory Liabilities [Member] | The Connecticut Light And Power Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	12,500	10,100
Other Regulatory Liabilities [Member] | NSTAR Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	2,200	28,900
Other Regulatory Liabilities [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	2,000	2,900
Other Regulatory Liabilities [Member] | Western Massachusetts Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	900	800
Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	1,980,000	2,016,000
Benefit Costs [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	443,300	445,400
Benefit Costs [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	496,100	515,900
Benefit Costs [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	175,100	174,300
Benefit Costs [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	85,900	85,000
Regulatory Assets Offsetting Derivative Liabilities [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	423,700	425,500
Regulatory Assets Offsetting Derivative Liabilities [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	420,200	410,900
Regulatory Assets Offsetting Derivative Liabilities [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	2,200	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	631,500	635,300
Deferred Income Tax Charges Member | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	438,900	437,700
Deferred Income Tax Charges Member | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	84,000	83,700
Deferred Income Tax Charges Member | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	34,800	38,000
Deferred Income Tax Charges Member | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	30,700	35,500
Storm Restoration Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	484,300	502,800
Storm Restoration Costs [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	298,400	319,600
Storm Restoration Costs [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	117,100	103,700
Storm Restoration Costs [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	41,100	47,700
Storm Restoration Costs [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	27,700	31,800
Goodwill Regulatory Asset [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	495,100	505,400
Goodwill Regulatory Asset [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	425,100	433,900
Regulatory Tracker Deferrals Regulatory Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	430,700	350,500
Regulatory Tracker Deferrals Regulatory Assets [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	44,800	16,100
Regulatory Tracker Deferrals Regulatory Assets [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	255,000	141,400
Regulatory Tracker Deferrals Regulatory Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	84,300	103,500
Regulatory Tracker Deferrals Regulatory Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	36,000	33,000
Unrecovered Contractual Obligations [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	124,800	123,800
Other Regulatory Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	156,300	167,300
Other Regulatory Assets [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	79,600	66,100
Other Regulatory Assets [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	77,200	94,700
Other Regulatory Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	35,700	41,300
Other Regulatory Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	$ 12,700	$ 12,900