SCHEDULE I - Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net Income	$ 176,600	$ 209,500	$ 46,200	$ 100,800	$ 114,800	$ 91,400	$ 78,700	$ 115,600	$ 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									(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
NSTAR Electric Company [Member]
Net Income	34,000	106,800	45,500	3,900	49,100	99,800	60,700	42,900	190,242	252,494	248,575
Nu Parent [Member]
Net Income									526,048	394,955	388,006
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									(2,168)	(27,316)	97
Comprehensive Income Attributable to Noncontrolling Interests									(103)	(262)	(57)
Comprehensive Income									$ 523,777	$ 367,377	$ 388,046