CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Shares [Member]	Capital Surplus, Paid In [Member]	Retained Earnings [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Loss [Member]	The Connecticut Light And Power Company [Member]	The Connecticut Light And Power Company [Member] Common Shares [Member]	The Connecticut Light And Power Company [Member] Capital Surplus, Paid In [Member]	The Connecticut Light And Power Company [Member] Retained Earnings [Member]	The Connecticut Light And Power Company [Member] Accumulated Other Comprehensive Loss [Member]	NSTAR Electric Company [Member]	NSTAR Electric Company [Member] Common Shares [Member]	NSTAR Electric Company [Member] Capital Surplus, Paid In [Member]	NSTAR Electric Company [Member] Retained Earnings [Member]	NSTAR Electric Company [Member] Accumulated Other Comprehensive Loss [Member]	Public Service Company Of New Hampshire [Member]	Public Service Company Of New Hampshire [Member] Common Shares [Member]	Public Service Company Of New Hampshire [Member] Capital Surplus, Paid In [Member]	Public Service Company Of New Hampshire [Member] Retained Earnings [Member]	Public Service Company Of New Hampshire [Member] Accumulated Other Comprehensive Loss [Member]	Western Massachusetts Electric Company [Member]	Western Massachusetts Electric Company [Member] Common Shares [Member]	Western Massachusetts Electric Company [Member] Capital Surplus, Paid In [Member]	Western Massachusetts Electric Company [Member] Retained Earnings [Member]	Western Massachusetts Electric Company [Member] Accumulated Other Comprehensive Loss [Member]
Beginning Balance (in shares) at Dec. 31, 2013		315,273,559						6,035,205					100					301					434,653
Beginning balance at Dec. 31, 2013	$ 9,611,528	$ 1,665,351	$ 6,192,765	$ 2,125,980	$ (326,537)	$ (46,031)	$ 2,702,494	$ 60,352	$ 1,682,047	$ 961,482	$ (1,387)	$ 2,413,453	$ 0	$ 992,625	$ 1,420,828	$ 0	$ 1,131,876	$ 0	$ 701,911	$ 438,515	$ (8,550)	$ 579,106	$ 10,866	$ 390,743	$ 181,014	$ (3,517)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	827,065			827,065			287,754			287,754		303,088			303,088		113,944			113,944		57,819			57,819
Dividends on Common Stock	(496,524)			(496,524)			(171,200)			(171,200)		(253,000)			(253,000)		(66,000)			(66,000)		(59,999)			(59,999)
Dividends on Preferred Stock	(7,519)			(7,519)			(5,559)			(5,559)		(1,961)			(1,961)
Allocation of Benefits - ESOP							2,771		2,771								1,329		1,329			513		513
Capital Stock Expenses, Net							51		51
Capital Contributions from Eversource Parent							120,000		120,000								45,000		45,000
Issuance of Common Shares (in shares)		288,941
Issuance of Common Shares	6,609	$ 1,445	5,164
Long-Term Incentive Plan Activity	(9,569)		(9,569)
Increase in Treasury Shares (in shares)		1,420,837
Increase in Treasury Shares	63,887		37,817		26,070
Other Changes in Shareholders' Equity	9,316		9,657	(341)								1,505		1,505
Other Comprehensive Income (Loss)	(27,978)					(27,978)	456				456	442				442	1,181				1,181	341				341
Ending Balance (in shares) at Dec. 31, 2014		316,983,337						6,035,205					100					301					434,653
Ending balance at Dec. 31, 2014	9,976,815	$ 1,666,796	6,235,834	2,448,661	(300,467)	(74,009)	2,936,767	$ 60,352	1,804,869	1,072,477	(931)	2,463,527	$ 0	994,130	1,468,955	442	1,227,330	$ 0	748,240	486,459	(7,369)	577,780	$ 10,866	391,256	178,834	(3,176)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	886,004			886,004			299,360			299,360		344,542			344,542		114,442			114,442		56,506			56,506
Dividends on Common Stock	(529,791)			(529,791)			(196,000)			(196,000)		(197,999)			(197,999)		(106,000)			(106,000)		(37,200)			(37,200)
Dividends on Preferred Stock	(7,519)			(7,519)			(5,559)			(5,559)		(1,960)			(1,960)
Allocation of Benefits - ESOP							743		743								394		394			142		142
Capital Stock Expenses, Net							51		51
Capital Contributions from Eversource Parent							105,000		105,000
Issuance of Common Shares (in shares)		503,443
Issuance of Common Shares	9,468	$ 2,517	6,951
Long-Term Incentive Plan Activity	(6,140)		(6,140)
Increase in Treasury Shares (in shares)		(295,531)
Increase in Treasury Shares	12,560		22,070		(9,510)
Other Changes in Shareholders' Equity	3,653		3,653									1,248		1,248
Other Comprehensive Income (Loss)	7,165					7,165	355				355	103				103	1,008				1,008	354
Ending Balance (in shares) at Dec. 31, 2015		317,191,249						6,035,205					100					301					434,653
Ending balance at Dec. 31, 2015	10,352,215	$ 1,669,313	6,262,368	2,797,355	(309,977)	(66,844)	3,140,717	$ 60,352	1,910,663	1,170,278	(576)	2,609,461	$ 0	995,378	1,613,538	545	1,237,174	$ 0	748,634	494,901	(6,361)	597,582	$ 10,866	391,398	198,140	(2,822)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	949,821			949,821			334,254					292,705					131,985					58,072
Dividends on Common Stock	(564,486)			(564,486)			(199,599)			(199,599)		(278,299)			(278,299)		(77,600)			(77,600)		(38,000)			(38,000)
Dividends on Preferred Stock	(7,519)			(7,519)			(5,559)			(5,559)		(1,960)			(1,960)
Capital Stock Expenses, Net							51		51
Capital Contributions from Eversource Parent							200,000		200,000			50,000		50,000			94,500		94,500			53,000		53,000
Issuance of Common Shares (in shares)		15,787
Issuance of Common Shares	(5,560)	$ 79	(5,639)
Long-Term Incentive Plan Activity	(6,056)		(6,056)
Increase in Treasury Shares (in shares)		(321,228)
Increase in Treasury Shares	(7,794)				(7,794)
Other Changes in Shareholders' Equity	(449)		(449)
Other Comprehensive Income (Loss)	1,562					1,562	523				523	(177)				(177)	1,298				1,298	460				460
Ending Balance (in shares) at Dec. 31, 2016		316,885,808						6,035,205					100					301					434,653
Ending balance at Dec. 31, 2016	$ 10,711,734	$ 1,669,392	$ 6,250,224	$ 3,175,171	$ (317,771)	$ (65,282)	$ 3,470,387	$ 60,352	$ 2,110,714	$ 1,299,374	$ (53)	$ 2,671,730	$ 0	$ 1,045,378	$ 1,625,984	$ 368	$ 1,387,357	$ 0	$ 843,134	$ 549,286	$ (5,063)	$ 671,114	$ 10,866	$ 444,398	$ 218,212	$ (2,362)