SCHEDULE I - Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Net Income	$ 179,200	$ 211,400	$ 173,100	$ 230,000	$ 176,600	$ 209,500	$ 46,200	$ 100,800	$ 793,689	$ 533,077	$ 400,513
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax									2,049	1,971	(14,177)
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax									(940)	217	506
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax									25,714	(4,356)	(13,645)
Other Comprehensive Income (Loss), Net of Tax									26,823	(2,168)	(27,316)
Comprehensive Income Attributable to Noncontrolling Interests									7,682	7,132	5,820
Comprehensive Income									812,830	523,777	367,377
NSTAR Electric Company [Member]
Net Income	55,400	107,000	58,000	48,100	34,000	106,800	45,500	3,900	268,546	190,242	252,494
Nu Parent [Member]
Net Income									786,007	526,048	394,955
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax									2,049	1,971	(14,177)
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax									(940)	217	506
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax									25,714	(4,356)	(13,645)
Other Comprehensive Income (Loss), Net of Tax									26,823	(2,168)	(27,316)
Comprehensive Income Attributable to Noncontrolling Interests									0	(103)	(262)
Comprehensive Income									$ 812,830	$ 523,777	$ 367,377