CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Net Income	$ 261,338	$ 246,033
Other Comprehensive (Loss)/Income, Net of Tax:
Qualified Cash Flow Hedging Instruments	534	534
Changes in Unrealized Gains/(Losses) on Marketable Securities	1,645	264
Changes in Funded Status of Pension, SERP and PBOP Benefit Plans	962	871
Net OCI	3,141	1,669
Comprehensive Income Attributable to Noncontrolling Interests	(1,880)	(1,880)
Comprehensive Income Attributable to Common Shareholders	262,599	245,822
The Connecticut Light And Power Company [Member]
Net Income	90,208	87,047
Other Comprehensive (Loss)/Income, Net of Tax:
Qualified Cash Flow Hedging Instruments	111	111
Changes in Unrealized Gains/(Losses) on Marketable Securities	56	9
Net OCI	167	120
Comprehensive Income Attributable to Common Shareholders	90,375	87,167
NSTAR Electric Company [Member]
Net Income	66,162	54,493
Other Comprehensive (Loss)/Income, Net of Tax:
Changes in Funded Status of Pension, SERP and PBOP Benefit Plans	(4)	(10)
Net OCI	(4)	(10)
Comprehensive Income Attributable to Common Shareholders	66,158	54,483
Public Service Company Of New Hampshire [Member]
Net Income	34,312	36,058
Other Comprehensive (Loss)/Income, Net of Tax:
Qualified Cash Flow Hedging Instruments	291	290
Changes in Unrealized Gains/(Losses) on Marketable Securities	97	16
Net OCI	388	306
Comprehensive Income Attributable to Common Shareholders	34,700	36,364
Western Massachusetts Electric Company [Member]
Net Income	17,218	16,827
Other Comprehensive (Loss)/Income, Net of Tax:
Qualified Cash Flow Hedging Instruments	109	109
Changes in Unrealized Gains/(Losses) on Marketable Securities	16	3
Net OCI	125	112
Comprehensive Income Attributable to Common Shareholders	$ 17,343	$ 16,939