CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Net Income	$ 255,133	$ 237,836
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	509	509
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	132	240
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	954	961
Other Comprehensive Income (Loss), Net of Tax, Total	1,595	1,710
Comprehensive Income Attributable to Noncontrolling Interests	(1,879)	(1,879)
Comprehensive Income	254,849	237,667
The Connecticut Light And Power Company [Member]
Net Income	69,234	79,299
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	111	111
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	4	8
Other Comprehensive Income (Loss), Net of Tax, Total	115	119
Comprehensive Income	69,349	79,418
NSTAR Electric Company [Member]
Net Income	83,561	58,087
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	(180)
Other Comprehensive Income (Loss), Net of Tax, Total	(180)
Comprehensive Income	83,381
Public Service Company Of New Hampshire [Member]
Net Income	32,045	32,638
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	291	290
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	8	14
Other Comprehensive Income (Loss), Net of Tax, Total	299	304
Comprehensive Income	32,344	32,942
Western Massachusetts Electric Company [Member]
Net Income	13,244	18,102
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	85	85
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	1	2
Other Comprehensive Income (Loss), Net of Tax, Total	86	87
Comprehensive Income	$ 13,330	$ 18,189