CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Shares	Capital Surplus, Paid In	Retained Earnings	Accumulated Other Comprehensive Loss	Treasury Stock	The Connecticut Light And Power Company	The Connecticut Light And Power Company Common Shares	The Connecticut Light And Power Company Capital Surplus, Paid In	The Connecticut Light And Power Company Retained Earnings	The Connecticut Light And Power Company Accumulated Other Comprehensive Loss	NSTAR Electric Company	NSTAR Electric Company Common Shares	NSTAR Electric Company Capital Surplus, Paid In	NSTAR Electric Company Retained Earnings	NSTAR Electric Company Accumulated Other Comprehensive Loss	Public Service Company Of New Hampshire	Public Service Company Of New Hampshire Common Shares	Public Service Company Of New Hampshire Capital Surplus, Paid In	Public Service Company Of New Hampshire Retained Earnings	Public Service Company Of New Hampshire Accumulated Other Comprehensive Loss
Beginning Balance (in shares) at Dec. 31, 2015		317,191,249						6,035,205					200					301
Beginning balance at Dec. 31, 2015	$ 10,352,215	$ 1,669,313	$ 6,262,368	$ 2,797,355	$ (66,844)	$ (309,977)	$ 3,140,717	$ 60,352	$ 1,910,663	$ 1,170,278	$ (576)	$ 3,207,044	$ 0	$ 1,397,642	$ 1,811,678	$ (2,276)	$ 1,237,174	$ 0	$ 748,634	$ 494,901	$ (6,361)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	949,821			949,821			334,254			334,254		350,777			350,777		131,985			131,985
Dividends on Common Stock	(564,486)			(564,486)			(199,599)			(199,599)		(316,300)			(316,300)		(77,600)			(77,600)
Dividends on Preferred Stock	(7,519)			(7,519)			(5,559)			(5,559)		(1,960)			(1,960)
Capital Contributions from Eversource Parent																	94,500		94,500
Capital Stock Expenses, Net							51		51
Capital Contributions from Eversource Parent							200,000		200,000
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)									103,000		103,000
Other Comprehensive Income	1,562				1,562		523				523	282				282	1,298				1,298
Ending Balance (in shares) at Dec. 31, 2016		316,885,808						6,035,205					200					301
Ending balance at Dec. 31, 2016	10,711,734	$ 1,669,392	6,250,224	3,175,171	(65,282)	(317,771)	3,470,387	$ 60,352	2,110,714	1,299,374	(53)	3,342,843	$ 0	1,500,642	1,844,195	(1,994)	1,387,357	$ 0	843,134	549,286	(5,063)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	995,515			995,515			376,726			376,726		374,726			374,726		135,996			135,996
Dividends on Common Stock	(602,083)			(602,083)			(254,800)			(254,800)		(272,000)			(272,000)		(173,900)			(173,900)
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												2,300		2,300
Long-Term Incentive Plan Activity	(10,834)		(10,834)
Other Changes in Shareholders' Equity	550		550
Other Comprehensive Income	(1,121)				(1,121)		322				322	171				171	1,141				1,141
Ending Balance (in shares) at Dec. 31, 2017		316,885,808						6,035,205					200					301
Ending balance at Dec. 31, 2017	11,086,242	$ 1,669,392	6,239,940	3,561,084	(66,403)	(317,771)	3,587,127	$ 60,352	2,110,765	1,415,741	269	3,446,080	$ 0	1,502,942	1,944,961	(1,823)	1,350,594	$ 0	843,134	511,382	(3,922)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	1,040,519			1,040,519			377,717					383,090			383,090		115,876			115,876
Dividends on Common Stock	(640,110)			(640,110)			(60,000)			(60,000)		(228,000)			(228,000)		(530,000)		(530,000)
Dividends on Preferred Stock	(7,519)			(7,519)			(5,559)			(5,559)		(1,960)			(1,960)
Capital Contributions from Eversource Parent																			365,000
Capital Contributions from Eversource Parent							300,000		300,000
Long-Term Incentive Plan Activity	(543)		(543)
Other Changes in Shareholders' Equity	1,825		1,825									130,500		130,500
Other Comprehensive Income	6,403				6,403		32				32	445				445	1,071				1,071
Ending Balance (in shares) at Dec. 31, 2018		316,885,808						6,035,205					200					301
Ending balance at Dec. 31, 2018	$ 11,486,817	$ 1,669,392	$ 6,241,222	$ 3,953,974	$ (60,000)	$ (317,771)	$ 4,199,317	$ 60,352	$ 2,410,765	$ 1,727,899	$ 301	$ 3,730,155	$ 0	$ 1,633,442	$ 2,098,091	$ (1,378)	$ 1,302,541	$ 0	$ 678,134	$ 627,258	$ (2,851)