REGULATORY ACCOUNTING (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Regulatory Asset [Line Items]
Regulatory Assets, Total	$ 4,622,800	$ 4,583,800
Regulatory Assets Current	919,311	845,843
Regulatory Assets Long Term	3,703,486	3,737,960
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	$ 76,100	75,300
Public Utilities Disclosure Of Regulatory Matters
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	$ 637,900	621,400
Regulatory Liability Current	111,414	107,759
Regulatory Liabilities Long-Term	526,452	513,595
The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	1,704,100	1,637,300
Regulatory Assets Current	324,559	268,318
Regulatory Assets Long Term	1,379,484	1,369,028
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	2,800	3,100
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	140,700	136,000
Regulatory Liability Current	62,999	61,155
Regulatory Liabilities Long-Term	77,744	74,830
NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	1,469,300	1,461,400
Regulatory Assets Current	361,307	348,408
Regulatory Assets Long Term	1,108,037	1,112,977
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	33,000	35,400
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	272,400	267,700
Regulatory Liability Current	4,997	3,281
Regulatory Liabilities Long-Term	267,440	264,352
Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	347,400	362,900
Regulatory Assets Current	101,633	104,971
Regulatory Assets Long Term	245,760	257,873
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	5,400	4,800
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	52,600	54,800
Regulatory Liability Current	4,723	6,898
Regulatory Liabilities Long-Term	47,930	47,851
Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	192,800	191,200
Regulatory Assets Current	61,363	56,166
Regulatory Assets Long Term	131,432	135,010
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	18,000	16,700
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	23,600	24,700
Regulatory Liability Current	9,760	13,122
Regulatory Liabilities Long-Term	13,775	11,597
Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	444,700	437,100
Cost Of Removal [Member] | The Connecticut Light And Power Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	26,700	24,100
Cost Of Removal [Member] | NSTAR Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	260,200	257,400
Cost Of Removal [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	47,300	47,200
Cost Of Removal [Member] | Western Massachusetts Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	3,700	2,800
Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	101,100	99,700
Regulatory Tracker Deferrals Regulatory Liabilities [Member] | The Connecticut Light And Power Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	49,700	56,200
Regulatory Tracker Deferrals Regulatory Liabilities [Member] | NSTAR Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	4,500	3,300
Regulatory Tracker Deferrals Regulatory Liabilities [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	2,600	3,400
Regulatory Tracker Deferrals Regulatory Liabilities [Member] | Western Massachusetts Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	10,900	12,900
AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	65,900	66,100
AFUDC Transmission Incentive [Member] | The Connecticut Light And Power Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	51,200	51,500
AFUDC Transmission Incentive [Member] | NSTAR Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	5,800	5,700
AFUDC Transmission Incentive [Member] | Western Massachusetts Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	8,900	8,900
Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	26,200	18,500
Other Regulatory Liabilities [Member] | The Connecticut Light And Power Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	13,100	4,200
Other Regulatory Liabilities [Member] | NSTAR Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	1,900	1,300
Other Regulatory Liabilities [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	2,700	4,200
Other Regulatory Liabilities [Member] | Western Massachusetts Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	100	100
Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	1,796,200	1,828,200
Benefit Costs [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	406,300	413,600
Benefit Costs [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	471,400	479,900
Benefit Costs [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	161,500	164,200
Benefit Costs [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	83,400	84,900
Regulatory Assets Offsetting Derivative Liabilities [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	389,600	388,000
Regulatory Assets Offsetting Derivative Liabilities [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	385,100	380,800
Regulatory Assets Offsetting Derivative Liabilities [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	3,900	1,300
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	646,600	650,900
Deferred Income Tax Charges Member | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	444,300	444,400
Deferred Income Tax Charges Member | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	85,700	85,700
Deferred Income Tax Charges Member | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	29,900	34,500
Deferred Income Tax Charges Member | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	31,300	31,800
Storm Restoration Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	450,500	436,900
Storm Restoration Costs [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	284,200	271,400
Storm Restoration Costs [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	118,300	110,900
Storm Restoration Costs [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	26,600	31,500
Storm Restoration Costs [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	21,400	23,100
Goodwill Regulatory Asset [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	479,800	484,900
Goodwill Regulatory Asset [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	411,900	416,300
Regulatory Tracker Deferrals Regulatory Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	604,400	526,500
Regulatory Tracker Deferrals Regulatory Assets [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	104,300	45,100
Regulatory Tracker Deferrals Regulatory Assets [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	325,900	311,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	98,100	101,200
Regulatory Tracker Deferrals Regulatory Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	45,300	40,100
Unrecovered Contractual Obligations [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	130,400	134,400
Other Regulatory Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	125,300	134,000
Other Regulatory Assets [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	79,900	82,000
Other Regulatory Assets [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	52,200	56,300
Other Regulatory Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	31,300	31,500
Other Regulatory Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	$ 11,400	$ 11,300