CONDENSED CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Adoption of New Accounting Standard	Common Shares	Capital Surplus, Paid In	Retained Earnings	Retained Earnings Adoption of New Accounting Standard	Accumulated Other Comprehensive Income (Loss)	Treasury Stock	The Connecticut Light and Power Company	The Connecticut Light and Power Company Adoption of New Accounting Standard	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 Retained Earnings Adoption of New Accounting Standard	The Connecticut Light and Power Company Accumulated Other Comprehensive Income (Loss)	NSTAR Electric Company	NSTAR Electric Company Adoption of New Accounting Standard	NSTAR Electric Company Common Shares	NSTAR Electric Company Capital Surplus, Paid In	NSTAR Electric Company Retained Earnings	NSTAR Electric Company Retained Earnings Adoption of New Accounting Standard	NSTAR Electric Company Accumulated Other Comprehensive Income (Loss)	Public Service Company of New Hampshire	Public Service Company of New Hampshire Adoption of New Accounting Standard	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 Retained Earnings Adoption of New Accounting Standard	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 Preferred Stock	(1,880)				(1,880)				(1,390)				(1,390)			(490)				(490)
Dividends on Common Shares	(169,757)				(169,757)				(99,000)				(99,000)			(60,600)				(60,600)			(19,000)				(19,000)
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, 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	664,655								326,962							338,688							100,512
Dividends on Preferred Stock	(5,600)
Other Comprehensive Income (Loss)	7,983								24							343							881
Balance (in shares) at Sep. 30, 2019			323,733,423								6,035,205							200							301
Balance at Sep. 30, 2019	12,121,614		$ 1,699,292	6,675,889	4,100,220		(52,017)	(301,770)	4,180,334		$ 60,352	2,410,765	1,708,892		325	3,915,916		$ 0	1,663,442	2,253,509		(1,035)	1,151,934		$ 0	678,134	475,770		(1,970)
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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	33,334				33,334				104,804				104,804			89,734				89,734			26,852				26,852
Dividends on Preferred Stock	(1,880)				(1,880)				(1,390)				(1,390)			(490)				(490)
Dividends on Common Shares	(169,857)				(169,857)				(176,400)				(176,400)			(121,200)				(121,200)			(214,000)				(214,000)
Issuance of Common Shares (in shares)			5,980,000
Issuance of Common Shares	433,550		$ 29,900	403,650
Long-Term Incentive Plan Activity	6,470			6,470
Issuance of Treasury Shares (in shares)			246,969
Issuance of Treasury Shares	18,027			13,448				4,579
Capital Stock Expense	(6,648)			(6,648)
Other									1				1
Other Comprehensive Income (Loss)	4,163						4,163		8						8	114						114	296						296
Balance (in shares) at Jun. 30, 2019			323,574,439								6,035,205							200							301
Balance at Jun. 30, 2019	11,954,593		$ 1,699,292	6,659,009	3,954,492		(53,641)	(304,559)	4,136,438		$ 60,352	2,410,765	1,664,995		326	3,751,354		$ 0	1,653,442	2,099,059		(1,147)	1,129,777		$ 0	678,134	453,891		(2,248)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	320,764				320,764				111,687				111,687			154,940				154,940			40,879				40,879
Dividends on Preferred Stock	(1,880)				(1,880)				(1,390)				(1,390)			(490)				(490)
Dividends on Common Shares	(173,156)				(173,156)				(66,400)				(66,400)										(19,000)				(19,000)
Long-Term Incentive Plan Activity	6,697			6,697
Issuance of Treasury Shares (in shares)			158,984
Issuance of Treasury Shares	12,972			10,183				2,789
Capital Contributions from Eversource Parent																10,000			10,000
Other Comprehensive Income (Loss)	1,624						1,624		(1)						(1)	112						112	278						278
Balance (in shares) at Sep. 30, 2019			323,733,423								6,035,205							200							301
Balance at Sep. 30, 2019	$ 12,121,614		$ 1,699,292	6,675,889	4,100,220		(52,017)	(301,770)	4,180,334		$ 60,352	2,410,765	1,708,892		325	3,915,916		$ 0	1,663,442	2,253,509		(1,035)	1,151,934		$ 0	678,134	475,770		(1,970)
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	$ (1,514)	$ 1,729,292	7,087,768	4,177,048	$ (1,514)	(65,059)	(299,055)	4,387,825	$ (900)	$ 60,352	2,535,765	1,791,392	$ (900)	316	4,159,884	$ (161)	$ 0	1,813,442	2,346,287	$ (161)	155	1,391,733	$ (300)	$ 0	903,134	490,306	$ (300)	(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 Preferred Stock	(1,880)				(1,880)				(1,390)				(1,390)			(490)				(490)
Dividends on Common Shares	(187,462)				(187,462)				(69,500)				(69,500)			(196,500)				(196,500)			(22,300)				(22,300)
Issuance of Common Shares (in shares)			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								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)
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	$ (1,514)	$ 1,729,292	7,087,768	4,177,048	$ (1,514)	(65,059)	(299,055)	4,387,825	$ (900)	$ 60,352	2,535,765	1,791,392	$ (900)	316	4,159,884	$ (161)	$ 0	1,813,442	2,346,287	$ (161)	155	1,391,733	$ (300)	$ 0	903,134	490,306	$ (300)	(1,707)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	938,883								356,106							363,585							118,384
Dividends on Preferred Stock	(5,600)
Other Comprehensive Income (Loss)	$ 4,662								(9)							200							823
Balance (in shares) at Sep. 30, 2020	342,797,473		342,797,473								6,035,205							200							301
Balance at Sep. 30, 2020	$ 13,980,017		$ 1,789,092	7,996,976	4,535,860		(60,397)	(281,514)	4,794,352		$ 60,352	2,660,765	2,072,928		307	4,325,539		$ 0	1,813,442	2,511,742		355	1,488,340		$ 0	903,134	586,090		(884)
Balance (in shares) at Mar. 31, 2020			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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	254,112				254,112				117,271				117,271			97,468				97,468			31,632				31,632
Dividends on Preferred Stock	(1,880)				(1,880)				(1,390)				(1,390)			(490)				(490)
Dividends on Common Shares	(190,964)				(190,964)
Issuance of Common Shares (in shares)			6,000,000
Issuance of Common Shares	517,560		$ 30,000	487,560
Long-Term Incentive Plan Activity	7,694			7,694
Issuance of Treasury Shares (in shares)			216,675
Issuance of Treasury Shares	16,548			12,524				4,024
Capital Stock Expense	(8,321)			(8,321)
Other Comprehensive Income (Loss)	553						553		3						3	68						68	285						285
Balance (in shares) at Jun. 30, 2020			342,627,862								6,035,205							200							301
Balance at Jun. 30, 2020	13,805,258		$ 1,789,092	7,979,146	4,384,093		(62,558)	(284,515)	4,550,656		$ 60,352	2,535,765	1,954,221		318	4,160,236		$ 0	1,813,442	2,346,504		290	1,440,929		$ 0	903,134	538,939		(1,144)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	348,140				348,140				120,097				120,097			165,728				165,728			47,151				47,151
Dividends on Preferred Stock	(1,880)				(1,880)				(1,390)				(1,390)			(490)				(490)
Dividends on Common Shares	(194,493)				(194,493)
Long-Term Incentive Plan Activity	6,921			6,921
Issuance of Treasury Shares (in shares)			169,611
Issuance of Treasury Shares	13,910			10,909				3,001
Capital Contributions from Eversource Parent									125,000			125,000
Other Comprehensive Income (Loss)	$ 2,161						2,161		(11)						(11)	65						65	260						260
Balance (in shares) at Sep. 30, 2020	342,797,473		342,797,473								6,035,205							200							301
Balance at Sep. 30, 2020	$ 13,980,017		$ 1,789,092	$ 7,996,976	$ 4,535,860		$ (60,397)	$ (281,514)	$ 4,794,352		$ 60,352	$ 2,660,765	$ 2,072,928		$ 307	$ 4,325,539		$ 0	$ 1,813,442	$ 2,511,742		$ 355	$ 1,488,340		$ 0	$ 903,134	$ 586,090		$ (884)