ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance	$ (66,844)	$ (74,000)
Other Comprehensive Income Before Reclassifications, Net of Tax	200	100
Reclassification From OCI, Net Of Tax	1,400	1,500
Other Comprehensive Income (Loss), Net of Tax, Total	1,669	1,595
AOCI Ending Balance	(65,175)	(72,400)
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense	(98,212)	(94,843)
Income Tax Expense	(146,302)	(153,226)
Net Income Attributable to Controlling Interests	244,153	253,254
The Connecticut Light And Power Company [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance	(576)
Other Comprehensive Income (Loss), Net of Tax, Total	120	115
AOCI Ending Balance	(456)
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense	(36,498)	(36,624)
Income Tax Expense	(48,863)	(38,147)
Public Service Company Of New Hampshire [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance	(6,361)
Other Comprehensive Income (Loss), Net of Tax, Total	306	299
AOCI Ending Balance	(6,055)
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense	(12,461)	(11,272)
Income Tax Expense	(22,326)	(20,276)
Western Massachusetts Electric Company [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance	(2,822)
Other Comprehensive Income (Loss), Net of Tax, Total	112	86
AOCI Ending Balance	(2,710)
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense	(6,004)	(6,823)
Income Tax Expense	(10,076)	(9,113)
Qualified Cash Flow Hedging Instruments [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance	(10,300)	(12,400)
Reclassification From OCI, Net Of Tax	500	500
Other Comprehensive Income (Loss), Net of Tax, Total	500	500
AOCI Ending Balance	(9,800)	(11,900)
Unrealized Gains Losses on Available for Sale Securities [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance	(1,900)	700
Other Comprehensive Income Before Reclassifications, Net of Tax	200	100
Other Comprehensive Income (Loss), Net of Tax, Total	200	100
AOCI Ending Balance	(1,700)	800
Pension, SERP and PBOP Benefit Plans [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance	(54,600)	(62,300)
Other Comprehensive Income Before Reclassifications, Net of Tax	0	0
Reclassification From OCI, Net Of Tax	900	1,000
Other Comprehensive Income (Loss), Net of Tax, Total	900	1,000
AOCI Ending Balance	$ (53,700)	$ (61,300)