CONDENSED CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Shares	Capital Surplus, Paid In	Retained Earnings	Accumulated Other Comprehensive Income (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 Income (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 Income (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 Income (Loss)
Balance (in shares) at Dec. 31, 2018		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 (Loss)	2,196				2,196		17				17	117				117	307				307
Balance (in shares) at Mar. 31, 2019		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)
Balance (in shares) at Dec. 31, 2019	329,880,645	329,880,645						6,035,205					200					301
Balance at Dec. 31, 2019	$ 12,629,994	$ 1,729,292	7,087,768	4,177,048	(65,059)	(299,055)	4,387,825	$ 60,352	2,535,765	1,791,392	316	4,159,884	$ 0	1,813,442	2,346,287	155	1,391,733	$ 0	903,134	490,306	(1,707)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	336,633			336,633			118,738			118,738		100,390			100,390		39,601			39,601
Dividends on Common Shares	(187,462)			(187,462)			(69,500)			(69,500)		(196,500)			(196,500)		(22,300)			(22,300)
Dividends on Preferred Stock	$ (1,880)			(1,880)			(1,390)			(1,390)		(490)			(490)
Issuance of Common Shares (in shares)	5,960,000	5,960,000
Issuance of Common Shares	$ 432,100	$ 29,800	402,300
Long-Term Incentive Plan Activity	(15,295)		(15,295)
Issuance of Treasury Shares (in shares)		570,542
Issuance of Treasury Shares	27,746		17,230			10,516
Capital Stock Expense	(12,314)		(12,314)
Other Comprehensive Income (Loss)	$ 1,948				1,948		(1)				(1)	67				67	278				278
Balance (in shares) at Mar. 31, 2020	336,411,187	336,411,187						6,035,205					200					301
Balance at Mar. 31, 2020	$ 13,209,956	$ 1,759,092	$ 7,479,689	$ 4,322,825	$ (63,111)	$ (288,539)	$ 4,434,772	$ 60,352	$ 2,535,765	$ 1,838,340	$ 315	$ 4,063,190	$ 0	$ 1,813,442	$ 2,249,526	$ 222	$ 1,409,012	$ 0	$ 903,134	$ 507,307	$ (1,429)