CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Net Income	$ 246,033	$ 255,133
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	534	509
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	264	132
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	871	954
Other Comprehensive Income (Loss), Net of Tax, Total	1,669	1,595
Comprehensive Income Attributable to Noncontrolling Interests	(1,880)	(1,879)
Comprehensive Income	245,822	254,849
The Connecticut Light And Power Company [Member]
Net Income	87,047	69,234
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	9	4
Other Comprehensive Income (Loss), Net of Tax, Total	120	115
Comprehensive Income	87,167	69,349
NSTAR Electric Company [Member]
Net Income	54,493	83,561
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	(10)	(180)
Other Comprehensive Income (Loss), Net of Tax, Total	(10)	(180)
Comprehensive Income	54,483	83,381
Public Service Company Of New Hampshire [Member]
Net Income	36,058	32,045
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	290	291
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	16	8
Other Comprehensive Income (Loss), Net of Tax, Total	306	299
Comprehensive Income	36,364	32,344
Western Massachusetts Electric Company [Member]
Net Income	16,827	13,244
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	109	85
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	3	1
Other Comprehensive Income (Loss), Net of Tax, Total	112	86
Comprehensive Income	$ 16,939	$ 13,330