CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Net Income	$ 237,799	$ 236,493	$ 702,319	$ 603,576
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	526	509	1,544	1,528
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	(2,803)	(216)	(3,919)	242
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	764	1,042	2,838	4,089
Other Comprehensive Income (Loss), Net of Tax, Total	(1,513)	1,335	463	5,859
Comprehensive Income Attributable to Noncontrolling Interests	(1,879)	(1,879)	(5,639)	(5,639)
Comprehensive Income	234,407	235,949	697,143	603,796
The Connecticut Light And Power Company [Member]
Net Income	80,188	83,876	228,192	200,560
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	111	111	333	333
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	(98)	(7)	(137)	8
Other Comprehensive Income (Loss), Net of Tax, Total	13	104	196	341
Comprehensive Income	80,201	83,980	228,388	200,901
NSTAR Electric Company [Member]
Net Income	118,624	115,574	284,148	233,791
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	(2)		(184)
Other Comprehensive Income (Loss), Net of Tax, Total	(2)		(184)
Comprehensive Income	118,622	115,574	283,964	233,791
Public Service Company Of New Hampshire [Member]
Net Income	32,501	28,236	92,491	84,955
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	291	290	872	871
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	(169)	(13)	(236)	14
Other Comprehensive Income (Loss), Net of Tax, Total	122	277	636	885
Comprehensive Income	32,623	28,513	93,127	85,840
Western Massachusetts Electric Company [Member]
Net Income	14,971	14,665	42,410	39,785
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	101	85	270	254
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	(27)	(2)	(38)	2
Other Comprehensive Income (Loss), Net of Tax, Total	74	83	232	256
Comprehensive Income	$ 15,045	$ 14,748	$ 42,642	$ 40,041