ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance			$ (74,009)	$ (46,000)
Other Comprehensive Income Before Reclassifications, Net of Tax			(4,300)	1,400
Reclassification From OCI, Net Of Tax			4,800	4,400
Other Comprehensive Income (Loss), Net of Tax, Total	$ (1,513)	$ 1,335	463	5,859
AOCI Ending Balance	(73,546)	(40,200)	(73,546)	(40,200)
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense	(92,534)	(89,738)	(279,635)	(272,208)
Income Tax Expense	(144,146)	(126,539)	(420,640)	(345,858)
Net Income Attributable to Controlling Interests	235,920	234,614	696,680	597,937
The Connecticut Light And Power Company [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance			(931)
Other Comprehensive Income (Loss), Net of Tax, Total	13	104	196	341
AOCI Ending Balance	(735)		(735)
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense	(36,716)	(38,735)	(109,463)	(110,448)
Income Tax Expense	(46,569)	(30,038)	(127,845)	(95,980)
Public Service Company Of New Hampshire [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance			(7,369)
Other Comprehensive Income (Loss), Net of Tax, Total	122	277	636	885
AOCI Ending Balance	(6,733)		(6,733)
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense	(11,647)	(11,024)	(34,582)	(33,995)
Income Tax Expense	(20,158)	(17,603)	(56,135)	(52,199)
Western Massachusetts Electric Company [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance			(3,176)
Other Comprehensive Income (Loss), Net of Tax, Total	74	83	232	256
AOCI Ending Balance	(2,944)		(2,944)
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense	(5,901)	(6,576)	(19,014)	(18,929)
Income Tax Expense	(9,749)	(10,490)	(28,555)	(26,596)
Qualified Cash Flow Hedging Instruments [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance			(12,400)	(14,400)
Reclassification From OCI, Net Of Tax			1,500	1,500
Other Comprehensive Income (Loss), Net of Tax, Total			1,500	1,500
AOCI Ending Balance	(10,900)	(12,900)	(10,900)	(12,900)
Unrealized Gains Losses on Available for Sale Securities [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance			700	400
Other Comprehensive Income Before Reclassifications, Net of Tax			(3,900)	200
Other Comprehensive Income (Loss), Net of Tax, Total			(3,900)	200
AOCI Ending Balance	(3,200)	600	(3,200)	600
Pension, SERP and PBOP Benefit Plans [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance			(62,300)	(32,000)
Other Comprehensive Income Before Reclassifications, Net of Tax			(400)	1,200
Reclassification From OCI, Net Of Tax			3,300	2,900
Other Comprehensive Income (Loss), Net of Tax, Total			2,900	4,100
AOCI Ending Balance	$ (59,400)	$ (27,900)	$ (59,400)	$ (27,900)