CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net Income	$ 533,077	$ 400,513	$ 394,107
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	1,971	(14,177)	200
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	217	506	402
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	(4,356)	(13,645)	(505)
Other Comprehensive Income (Loss), Net of Tax, Total	(2,168)	(27,316)	97
Comprehensive Income Attributable to Noncontrolling Interests	(7,132)	(5,820)	(6,158)
Comprehensive Income	523,777	367,377	388,046
The Connecticut Light And Power Company [Member]
Net Income	209,725	250,164	244,143
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	444	445	444
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	7	17	14
Other Comprehensive Income (Loss), Net of Tax, Total	451	462	458
Comprehensive Income	210,176	250,626	244,601
Public Service Company Of New Hampshire [Member]
Net Income	96,882	100,267	90,067
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	1,162	(10,260)	87
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	13	29	24
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	2
Other Comprehensive Income (Loss), Net of Tax, Total	1,177	(10,231)	111
Comprehensive Income	98,059	90,036	90,178
Western Massachusetts Electric Company [Member]
Net Income	54,503	43,054	23,090
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	338	(4,108)	(79)
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	2	5	4
Other Comprehensive Income (Loss), Net of Tax, Total	340	(4,103)	(75)
Comprehensive Income	54,843	38,951	23,015
NSTAR Electric Company [Member]
Net Income	$ 190,242	$ 252,494	$ 248,575