ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance				$ (46,031,000)				$ (72,900,000)	$ (46,031,000)	$ (72,900,000)
Other Comprehensive Income Before Reclassifications, Net of Tax									(33,900,000)	18,500,000
Reclassification From OCI, Net Of Tax									5,900,000	8,400,000
Other Comprehensive Income (Loss), Net of Tax, Total									(27,978,000)	26,823,000	(2,168,000)
AOCI Ending Balance	(74,009,000)				(46,031,000)				(74,009,000)	(46,031,000)	(72,900,000)
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense									(362,106,000)	(338,699,000)	(329,945,000)
Income Tax Expense									(468,297,000)	(426,941,000)	(274,926,000)
Net Income Attributable to Controlling Interests	221,500,000	234,600,000	127,400,000	236,000,000	177,400,000	209,500,000	171,000,000	228,100,000	819,546,000	786,007,000	525,945,000
OCI Benefit Plans Before Reclassification Adjustments, Tax [Abstract]
OCI Benefit Plans Before Reclassification Adjustments TAX Effect									18,700,000	11,400,000	6,200,000
Future AOCI Reclassification Amounts [Abstract]
Cash Flow Hedge Loss To Be Reclassified Within Twelve Months									3,500,000
Pension SERP And PBOP Gain Loss To Be Reclassified Within Twelve Months									6,900,000
The Connecticut Light And Power Company [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance				(1,387,000)					(1,387,000)
Other Comprehensive Income (Loss), Net of Tax, Total									456,000	413,000	451,000
AOCI Ending Balance	(931,000)				(1,387,000)				(931,000)	(1,387,000)
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense									(147,421,000)	(133,650,000)	(133,127,000)
Income Tax Expense									(133,451,000)	(141,663,000)	(94,437,000)
Future AOCI Reclassification Amounts [Abstract]
Cash Flow Hedge Loss To Be Reclassified Within Twelve Months									700,000
Public Service Company Of New Hampshire [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance				(8,550,000)					(8,550,000)
Other Comprehensive Income (Loss), Net of Tax, Total									1,181,000	1,105,000	1,177,000
AOCI Ending Balance	(7,369,000)				(8,550,000)				(7,369,000)	(8,550,000)
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense									(45,349,000)	(46,176,000)	(50,228,000)
Income Tax Expense									(72,135,000)	(71,101,000)	(60,993,000)
Future AOCI Reclassification Amounts [Abstract]
Cash Flow Hedge Loss To Be Reclassified Within Twelve Months									2,000,000
Western Massachusetts Electric Company [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance				(3,517,000)					(3,517,000)
Other Comprehensive Income (Loss), Net of Tax, Total									341,000	329,000	340,000
AOCI Ending Balance	(3,176,000)				(3,517,000)				(3,176,000)	(3,517,000)
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense									(24,931,000)	(24,851,000)	(26,634,000)
Income Tax Expense									(37,268,000)	(37,368,000)	(32,140,000)
Future AOCI Reclassification Amounts [Abstract]
Cash Flow Hedge Loss To Be Reclassified Within Twelve Months									600,000
Qualified Cash Flow Hedging Instruments [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance				(14,400,000)				(16,400,000)	(14,400,000)	(16,400,000)
Other Comprehensive Income Before Reclassifications, Net of Tax									0
Reclassification From OCI, Net Of Tax									2,000,000	2,000,000
Other Comprehensive Income (Loss), Net of Tax, Total									2,000,000	2,000,000
AOCI Ending Balance	(12,400,000)				(14,400,000)				(12,400,000)	(14,400,000)
Unrealized Gains Losses on Available for Sale Securities [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance				400,000				1,300,000	400,000	1,300,000
Other Comprehensive Income Before Reclassifications, Net of Tax									300,000	(900,000)
Other Comprehensive Income (Loss), Net of Tax, Total									300,000	(900,000)
AOCI Ending Balance	700,000				400,000				700,000	400,000
Pension, SERP and PBOP Benefit Plans [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance				(32,000,000)				(57,800,000)	(32,000,000)	(57,800,000)
Other Comprehensive Income Before Reclassifications, Net of Tax									(34,200,000)	19,400,000
Reclassification From OCI, Net Of Tax									3,900,000	6,400,000
Other Comprehensive Income (Loss), Net of Tax, Total									(30,300,000)	25,800,000
AOCI Ending Balance	(62,300,000)				(32,000,000)				(62,300,000)	(32,000,000)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Net Income Attributable to Controlling Interests									(5,900,000)	(8,400,000)	(7,800,000)
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									(3,400,000)	(3,400,000)	(3,300,000)
Income Tax Expense									1,400,000	1,400,000	1,300,000
Net Income Attributable to Controlling Interests									(2,000,000)	(2,000,000)	(2,000,000)
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									(6,200,000)	(10,500,000)	(8,900,000)
OCI Reclassification for Transition Obligation											(200,000)
OCI Reclassification Adjustment for Amortization of Prior Service Cost, before tax									(200,000)	(200,000)	(200,000)
OCI Reclassification Adjustment for Pension, SERP and PBOP Plans, before Tax									(6,400,000)	(10,700,000)	(9,300,000)
Income Tax Expense									2,500,000	4,300,000	3,500,000
Net Income Attributable to Controlling Interests									$ (3,900,000)	$ (6,400,000)	$ (5,800,000)