CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Net Income	$ 827,065	$ 793,689	$ 533,077
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	2,037	2,049	1,971
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	315	(940)	217
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	(30,330)	25,714	(4,356)
Other Comprehensive Income (Loss), Net of Tax, Total	(27,978)	26,823	(2,168)
Comprehensive Income Attributable to Noncontrolling Interests	7,519	7,682	7,132
Comprehensive Income	791,568	812,830	523,777
The Connecticut Light And Power Company [Member]
Net Income	287,754	279,412	209,725
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	444	444	444
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	12	(31)	7
Other Comprehensive Income (Loss), Net of Tax, Total	456	413	451
Comprehensive Income	288,210	279,825	210,176
NSTAR Electric Company [Member]
Net Income	303,088	268,546	190,242
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	442
Other Comprehensive Income (Loss), Net of Tax, Total	442
Comprehensive Income	303,530	268,546	190,242
Public Service Company Of New Hampshire [Member]
Net Income	113,944	111,397	96,882
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	1,162	1,162	1,162
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	19	(54)	13
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	0	(3)	2
Other Comprehensive Income (Loss), Net of Tax, Total	1,181	1,105	1,177
Comprehensive Income	115,125	112,502	98,059
Western Massachusetts Electric Company [Member]
Net Income	57,819	60,438	54,503
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	338	338	338
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	3	(9)	2
Other Comprehensive Income (Loss), Net of Tax, Total	341	329	340
Comprehensive Income	$ 58,160	$ 60,767	$ 54,843