ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance				$ (74,009)				$ (46,000)	$ (74,009)	$ (46,000)
Other Comprehensive Income Before Reclassifications, Net of Tax									900	(33,900)
Reclassification From OCI, Net Of Tax									6,300	5,900
Other Comprehensive Income (Loss), Net of Tax, Total									7,165	(27,978)	$ 26,823
AOCI Ending Balance	$ (66,844)				$ (74,009)				(66,844)	(74,009)	(46,000)
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense									(372,420)	(362,106)	(338,699)
Income Tax Expense									(539,967)	(468,297)	(426,941)
Net Income Attributable to Controlling Interests	181,800	$ 235,900	$ 207,500	253,300	221,500	$ 234,600	$ 127,400	236,000	878,485	819,546	786,007
OCI Benefit Plans Before Reclassification Adjustments, Tax [Abstract]
OCI Benefit Plans Before Reclassification Adjustments TAX Effect									2,000	(22,300)	11,400
Future AOCI Reclassification Amounts [Abstract]
Cash Flow Hedge Loss To Be Reclassified Within Twelve Months									3,500
Pension SERP And PBOP Gain Loss To Be Reclassified Within Twelve Months									6,900
The Connecticut Light And Power Company [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance				(931)					(931)
Other Comprehensive Income (Loss), Net of Tax, Total									355	456	413
AOCI Ending Balance	(576)				(931)				(576)	(931)
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense									(145,795)	(147,421)	(133,650)
Income Tax Expense									(177,396)	(133,451)	(141,663)
Future AOCI Reclassification Amounts [Abstract]
Cash Flow Hedge Loss To Be Reclassified Within Twelve Months									700
Public Service Company Of New Hampshire [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance				(7,369)					(7,369)
Other Comprehensive Income (Loss), Net of Tax, Total									1,008	1,181	1,105
AOCI Ending Balance	(6,361)				(7,369)				(6,361)	(7,369)
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense									(45,990)	(45,349)	(46,176)
Income Tax Expense									(73,060)	(72,135)	(71,101)
Future AOCI Reclassification Amounts [Abstract]
Cash Flow Hedge Loss To Be Reclassified Within Twelve Months									2,000
Western Massachusetts Electric Company [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance				(3,176)					(3,176)
Other Comprehensive Income (Loss), Net of Tax, Total									354	341	329
AOCI Ending Balance	(2,822)				(3,176)				(2,822)	(3,176)
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense									(24,792)	(24,931)	(24,851)
Income Tax Expense									(36,970)	(37,268)	(37,368)
Future AOCI Reclassification Amounts [Abstract]
Cash Flow Hedge Loss To Be Reclassified Within Twelve Months									600
Qualified Cash Flow Hedging Instruments [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance				(12,400)				(14,400)	(12,400)	(14,400)
Other Comprehensive Income Before Reclassifications, Net of Tax									0
Reclassification From OCI, Net Of Tax									2,100	2,000
Other Comprehensive Income (Loss), Net of Tax, Total									2,100	2,000
AOCI Ending Balance	(10,300)				(12,400)				(10,300)	(12,400)	(14,400)
Unrealized Gains Losses on Available for Sale Securities [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance				700				400	700	400
Other Comprehensive Income Before Reclassifications, Net of Tax									(2,600)	300
Other Comprehensive Income (Loss), Net of Tax, Total									(2,600)	300
AOCI Ending Balance	(1,900)				700				(1,900)	700	400
Pension, SERP and PBOP Benefit Plans [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance				$ (62,300)				$ (32,000)	(62,300)	(32,000)
Other Comprehensive Income Before Reclassifications, Net of Tax									3,500	(34,200)
Reclassification From OCI, Net Of Tax									4,200	3,900
Other Comprehensive Income (Loss), Net of Tax, Total									7,700	(30,300)
AOCI Ending Balance	$ (54,600)				$ (62,300)				(54,600)	(62,300)	(32,000)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Net Income Attributable to Controlling Interests									(6,300)	(5,900)	(8,400)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Qualified Cash Flow Hedging Instruments [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense									(3,500)	(3,400)	(3,400)
Income Tax Expense									1,400	1,400	1,400
Net Income Attributable to Controlling Interests									(2,100)	(2,000)	(2,000)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Pension, SERP and PBOP Benefit Plans [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
OCI Reclassification Adjustment for Net Actuarial Gain (Loss), before Tax									(6,600)	(6,200)	(10,500)
OCI Reclassification Adjustment for Amortization of Prior Service Cost, before tax									(200)	(200)	(200)
OCI Reclassification Adjustment for Pension, SERP and PBOP Plans, before Tax									(6,800)	(6,400)	(10,700)
Income Tax Expense									2,600	2,500	4,300
Net Income Attributable to Controlling Interests									$ (4,200)	$ (3,900)	$ (6,400)