CONDENSED 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
Balance (in shares) at Dec. 31, 2017		316,885,808						6,035,205					200					301
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	271,426			271,426			98,568			98,568		77,149			77,149		35,093			35,093
Dividends on Common Shares	(160,027)			(160,027)			(60,000)			(60,000)		(161,000)			(161,000)
Dividends on Preferred Stock	(1,880)			(1,880)			(1,390)			(1,390)		(490)			(490)
Other												1			1
Long-Term Incentive Plan Activity	(15,320)		(15,320)
Capital Contributions from Eversource Parent							9,000		9,000			92,500		92,500
Other Comprehensive Income	3,273				3,273		40				40	106				106	269				269
Balance (in shares) at Mar. 31, 2018		316,885,808						6,035,205					200					301
Balance at Mar. 31, 2018	$ 11,183,714	$ 1,669,392	6,224,620	3,670,603	(63,130)	(317,771)	3,633,345	$ 60,352	2,119,765	1,452,919	309	3,454,346	$ 0	1,595,442	1,860,621	(1,717)	1,385,956	$ 0	843,134	546,475	(3,653)
Balance (in shares) at Dec. 31, 2018	316,885,808	316,885,808						6,035,205					200					301
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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	310,558			310,558			110,471			110,471		94,014			94,014		32,781			32,781
Dividends on Common Shares	(169,757)			(169,757)			(99,000)			(99,000)		(60,600)			(60,600)		(19,000)			(19,000)
Dividends on Preferred Stock	(1,880)			(1,880)			(1,390)			(1,390)		(490)			(490)
Long-Term Incentive Plan Activity	(16,609)		(16,609)
Issuance of Treasury Shares (in shares)		461,662
Issuance of Treasury Shares	26,109		17,476			8,633
Capital Contributions from Eversource Parent												20,000		20,000
Other Comprehensive Income	$ 2,196				2,196		17				17	117				117	307				307
Balance (in shares) at Mar. 31, 2019	317,347,470	317,347,470						6,035,205					200					301
Balance at Mar. 31, 2019	$ 11,637,434	$ 1,669,392	$ 6,242,089	$ 4,092,895	$ (57,804)	$ (309,138)	$ 4,209,415	$ 60,352	$ 2,410,765	$ 1,737,980	$ 318	$ 3,783,196	$ 0	$ 1,653,442	$ 2,131,015	$ (1,261)	$ 1,316,629	$ 0	$ 678,134	$ 641,039	$ (2,544)