REGULATORY ACCOUNTING (Details) - USD ($) $ in Thousands	3 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Regulatory Asset [Line Items]
Regulatory Assets, Total	$ 4,605,700	$ 4,726,600
Regulatory Assets Current	653,892	672,493
Regulatory Assets Long Term	3,951,752	4,054,086
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	$ 59,400	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 quarter 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. 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 of 2015, 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. NSTAR Electric requested recovery from customers effective January 1, 2016 in briefs filed with the DPU in October 2015. On October 27, 2015, the DPU delayed the effective date of the rate increase from November 1, 2015 to December 1, 2015 to allow the DPU staff additional time to review the reconciliations. NSTAR Electric expects a decision from the DPU in the fourth quarter of 2015. CL&P Distribution Rates: On July 2, 2015, PURA issued a final order that 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 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, $15.3 million has been recorded as a regulatory asset through September 30, 2015, with a corresponding increase in Operating Revenues.
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	$ 743,700	750,100
Regulatory Liability Current	226,061	235,022
Regulatory Liabilities Long-Term	517,595	515,144
The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	1,664,300	1,695,800
Regulatory Assets Current	243,293	220,344
Regulatory Assets Long Term	1,420,974	1,475,508
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	2,400	1,300
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	213,300	206,000
Regulatory Liability Current	136,393	124,722
Regulatory Liabilities Long-Term	76,864	81,298
NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	1,403,500	1,377,800
Regulatory Assets Current	223,520	198,710
Regulatory Assets Long Term	1,179,996	1,179,100
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	21,100	33,200
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	279,600	312,300
Regulatory Liability Current	12,319	49,611
Regulatory Liabilities Long-Term	267,342	262,738
Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	361,900	404,800
Regulatory Assets Current	89,513	111,705
Regulatory Assets Long Term	272,360	293,115
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	3,200	900
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	61,000	67,400
Regulatory Liability Current	11,338	16,044
Regulatory Liabilities Long-Term	49,680	51,372
Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	182,500	198,200
Regulatory Assets Current	48,805	51,923
Regulatory Assets Long Term	133,697	146,307
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	16,200	11,000
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	36,200	33,300
Regulatory Liability Current	24,267	22,486
Regulatory Liabilities Long-Term	11,926	10,835
Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	433,700	439,900
Cost Of Removal [Member] | The Connecticut Light And Power Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	17,200	19,700
Cost Of Removal [Member] | NSTAR Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	257,700	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	2,000	1,100
Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	215,900	192,300
Regulatory Tracker Deferrals Regulatory Liabilities [Member] | The Connecticut Light And Power Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	131,100	122,600
Regulatory Tracker Deferrals Regulatory Liabilities [Member] | NSTAR Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	13,000	20,700
Regulatory Tracker Deferrals Regulatory Liabilities [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	8,600	14,200
Regulatory Tracker Deferrals Regulatory Liabilities [Member] | Western Massachusetts Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	24,200	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	51,800	53,600
AFUDC Transmission Incentive [Member] | NSTAR Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	5,400	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	27,900	50,800
Other Regulatory Liabilities [Member] | The Connecticut Light And Power Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	13,200	10,100
Other Regulatory Liabilities [Member] | NSTAR Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	3,500	28,900
Other Regulatory Liabilities [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	3,300	2,900
Other Regulatory Liabilities [Member] | Western Massachusetts Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	1,000	800
Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	1,941,800	2,016,000
Benefit Costs [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	434,800	445,400
Benefit Costs [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	486,800	515,900
Benefit Costs [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	171,100	174,300
Benefit Costs [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	84,200	85,000
Regulatory Assets Offsetting Derivative Liabilities [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	412,800	425,500
Regulatory Assets Offsetting Derivative Liabilities [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	406,900	410,900
Regulatory Assets Offsetting Derivative Liabilities [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	1,400	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	635,800	635,300
Deferred Income Tax Charges Member | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	435,700	437,700
Deferred Income Tax Charges Member | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	83,500	83,700
Deferred Income Tax Charges Member | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	34,200	38,000
Deferred Income Tax Charges Member | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	30,500	35,500
Storm Restoration Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	461,300	502,800
Storm Restoration Costs [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	286,500	319,600
Storm Restoration Costs [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	113,500	103,700
Storm Restoration Costs [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	36,300	47,700
Storm Restoration Costs [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	25,000	31,800
Goodwill Regulatory Asset [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	490,000	505,400
Goodwill Regulatory Asset [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	420,700	433,900
Regulatory Tracker Deferrals Regulatory Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	373,100	350,500
Regulatory Tracker Deferrals Regulatory Assets [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	16,000	16,100
Regulatory Tracker Deferrals Regulatory Assets [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	228,000	141,400
Regulatory Tracker Deferrals Regulatory Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	85,400	103,500
Regulatory Tracker Deferrals Regulatory Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	30,100	33,000
Unrecovered Contractual Obligations [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	142,000	123,800
Other Regulatory Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	148,900	167,300
Other Regulatory Assets [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	84,400	66,100
Other Regulatory Assets [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	69,600	94,700
Other Regulatory Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	34,900	41,300
Other Regulatory Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	$ 12,700	$ 12,900