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SCHEDULE I - Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	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
Net Income	$ 223,600us-gaap_NetIncomeLoss	$ 236,500us-gaap_NetIncomeLoss	$ 129,200us-gaap_NetIncomeLoss	$ 237,800us-gaap_NetIncomeLoss	$ 179,200us-gaap_NetIncomeLoss	$ 211,400us-gaap_NetIncomeLoss	$ 173,100us-gaap_NetIncomeLoss	$ 230,000us-gaap_NetIncomeLoss	$ 827,065us-gaap_NetIncomeLoss	$ 793,689us-gaap_NetIncomeLoss	$ 533,077us-gaap_NetIncomeLoss
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									(27,978)	26,823	(2,168)
Comprehensive Income Attributable to Noncontrolling Interests									(7,519)	(7,682)	(7,132)
Comprehensive Income									791,568us-gaap_ComprehensiveIncomeNetOfTax	812,830us-gaap_ComprehensiveIncomeNetOfTax	523,777us-gaap_ComprehensiveIncomeNetOfTax
NSTAR Electric Company [Member]
Net Income	69,300	115,600	60,100	58,100	55,400	107,000	58,000	48,100	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									442
Comprehensive Income									303,530	268,546	190,242
Nu Parent [Member]
Net Income									819,546	786,007	526,048
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									(27,978)	26,823	(2,168)
Comprehensive Income Attributable to Noncontrolling Interests									0	0	(103)
Comprehensive Income									$ 791,568	$ 812,830	$ 523,777