REGULATORY ACCOUNTING (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Regulatory Asset [Line Items]
Regulatory Assets, Total	$ 4,525,000	$ 4,583,800
Regulatory Assets Current	863,360	845,843
Regulatory Assets Long Term	3,661,578	3,737,960
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	$ 83,000	75,300
Public Utilities Disclosure Of Regulatory Matters
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	$ 647,100	621,400
Regulatory Liability Current	119,413	107,759
Regulatory Liabilities Long-Term	527,678	513,595
The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	1,681,100	1,637,300
Regulatory Assets Current	312,160	268,318
Regulatory Assets Long Term	1,368,897	1,369,028
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	2,600	3,100
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	143,900	136,000
Regulatory Liability Current	63,287	61,155
Regulatory Liabilities Long-Term	80,617	74,830
NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	1,428,800	1,461,400
Regulatory Assets Current	316,715	348,408
Regulatory Assets Long Term	1,112,099	1,112,977
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	38,500	35,400
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	274,700	267,700
Regulatory Liability Current	6,923	3,281
Regulatory Liabilities Long-Term	267,809	264,352
Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	361,500	362,900
Regulatory Assets Current	122,879	104,971
Regulatory Assets Long Term	238,612	257,873
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	5,500	4,800
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	53,300	54,800
Regulatory Liability Current	5,986	6,898
Regulatory Liabilities Long-Term	47,283	47,851
Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	182,500	191,200
Regulatory Assets Current	57,660	56,166
Regulatory Assets Long Term	124,804	135,010
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	19,400	16,700
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	24,600	24,700
Regulatory Liability Current	11,256	13,122
Regulatory Liabilities Long-Term	13,280	11,597
Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	448,500	437,100
Cost Of Removal [Member] | The Connecticut Light And Power Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	30,400	24,100
Cost Of Removal [Member] | NSTAR Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	261,600	257,400
Cost Of Removal [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	46,600	47,200
Cost Of Removal [Member] | Western Massachusetts Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	4,600	2,800
Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	104,400	99,700
Regulatory Tracker Deferrals Regulatory Liabilities [Member] | The Connecticut Light And Power Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	50,000	56,200
Regulatory Tracker Deferrals Regulatory Liabilities [Member] | NSTAR Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	6,100	3,300
Regulatory Tracker Deferrals Regulatory Liabilities [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	3,200	3,400
Regulatory Tracker Deferrals Regulatory Liabilities [Member] | Western Massachusetts Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	11,100	12,900
AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	65,700	66,100
AFUDC Transmission Incentive [Member] | The Connecticut Light And Power Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	50,900	51,500
AFUDC Transmission Incentive [Member] | NSTAR Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	6,000	5,700
AFUDC Transmission Incentive [Member] | Western Massachusetts Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	8,800	8,900
Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	28,500	18,500
Other Regulatory Liabilities [Member] | The Connecticut Light And Power Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	12,600	4,200
Other Regulatory Liabilities [Member] | NSTAR Electric Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	1,000	1,300
Other Regulatory Liabilities [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	3,500	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	1,754,100	1,828,200
Benefit Costs [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	383,200	413,600
Benefit Costs [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	477,000	479,900
Benefit Costs [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	160,000	164,200
Benefit Costs [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	77,900	84,900
Regulatory Assets Offsetting Derivative Liabilities [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	413,700	388,000
Regulatory Assets Offsetting Derivative Liabilities [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	411,300	380,800
Regulatory Assets Offsetting Derivative Liabilities [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	2,400	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	644,300	650,900
Deferred Income Tax Charges Member | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	442,500	444,400
Deferred Income Tax Charges Member | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	86,300	85,700
Deferred Income Tax Charges Member | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	28,200	34,500
Deferred Income Tax Charges Member | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	31,000	31,800
Storm Restoration Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	428,900	436,900
Storm Restoration Costs [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	269,500	271,400
Storm Restoration Costs [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	117,700	110,900
Storm Restoration Costs [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	21,700	31,500
Storm Restoration Costs [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	20,000	23,100
Goodwill Regulatory Asset [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	474,600	484,900
Goodwill Regulatory Asset [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	407,500	416,300
Regulatory Tracker Deferrals Regulatory Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	562,500	526,500
Regulatory Tracker Deferrals Regulatory Assets [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	96,600	45,100
Regulatory Tracker Deferrals Regulatory Assets [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	287,000	311,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	120,500	101,200
Regulatory Tracker Deferrals Regulatory Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	41,500	40,100
Unrecovered Contractual Obligations [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	117,300	134,400
Other Regulatory Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	129,600	134,000
Other Regulatory Assets [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	78,000	82,000
Other Regulatory Assets [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	50,900	56,300
Other Regulatory Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	31,100	31,500
Other Regulatory Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	$ 12,100	$ 11,300