ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 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			(1,300)	(1,500)
Reclassification From OCI, Net Of Tax			2,700	3,500
Other Comprehensive Income (Loss), Net of Tax, Total	$ (189)	$ 381	1,480	1,976
AOCI Ending Balance	(65,364)	(72,000)	(65,364)	(72,000)
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense	(100,492)	(92,259)	(198,703)	(187,102)
Income Tax Expense	(125,439)	(123,268)	(271,742)	(276,494)
Net Income Attributable to Controlling Interests	203,649	207,507	447,803	460,761
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	147	68	267	183
AOCI Ending Balance	(309)		(309)
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense	(35,978)	(36,124)	(72,477)	(72,748)
Income Tax Expense	(49,563)	(43,129)	(98,427)	(81,276)
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	354	215	660	514
AOCI Ending Balance	(5,701)		(5,701)
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense	(12,526)	(11,662)	(24,987)	(22,935)
Income Tax Expense	(19,573)	(15,701)	(41,899)	(35,977)
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	120	72	232	158
AOCI Ending Balance	(2,590)		(2,590)
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense	(6,072)	(6,291)	(12,075)	(13,112)
Income Tax Expense	(9,995)	(9,693)	(20,071)	(18,807)
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			1,100	1,000
Other Comprehensive Income (Loss), Net of Tax, Total			1,100	1,000
AOCI Ending Balance	(9,200)	(11,400)	(9,200)	(11,400)
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			1,300	(1,100)
Other Comprehensive Income (Loss), Net of Tax, Total			1,300	(1,100)
AOCI Ending Balance	(600)	(400)	(600)	(400)
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			(2,600)	(400)
Reclassification From OCI, Net Of Tax			1,600	2,500
Other Comprehensive Income (Loss), Net of Tax, Total			(1,000)	2,100
AOCI Ending Balance	$ (55,600)	$ (60,200)	$ (55,600)	$ (60,200)