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 (Loss)/Income	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 (Loss)/Income	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 (Loss)/Income	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 (Loss)/Income
Balance (in shares) at Dec. 31, 2019			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 Stock	(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								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								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 Stock	(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 Stock	(194,493)				(194,493)
Capital Contributions from Eversource Parent									125,000			125,000
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
Other Comprehensive Income (Loss)	2,161						2,161		(11)						(11)	65						65	260						260
Balance (in shares) at Sep. 30, 2020			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 Dec. 31, 2020	342,954,023		342,954,023								6,035,205							200							301
Balance at Dec. 31, 2020	$ 14,063,566		$ 1,789,092	8,015,663	4,613,201		(76,411)	(277,979)	5,044,786		$ 60,352	2,810,765	2,173,367		302	4,521,418		$ 0	1,993,942	2,527,167		309	1,542,539		$ 0	928,134	615,018		(613)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	368,023				368,023				98,398				98,398			93,924				93,924			44,676				44,676
Dividends on Preferred Stock	(1,880)				(1,880)				(1,390)				(1,390)			(490)				(490)
Dividends on Common Stock	(206,913)				(206,913)				(70,100)				(70,100)			(206,400)				(206,400)			(25,200)				(25,200)
Long-Term Incentive Plan Activity	(15,727)			(15,727)
Issuance of Treasury Shares (in shares)			480,275
Issuance of Treasury Shares	25,163			16,182				8,981
Other Comprehensive Income (Loss)	1,188						1,188		(32)						(32)	61						61	255						255
Balance (in shares) at Mar. 31, 2021			343,434,298								6,035,205							200							301
Balance at Mar. 31, 2021	$ 14,233,420		$ 1,789,092	8,016,118	4,772,431		(75,223)	(268,998)	5,071,662		$ 60,352	2,810,765	2,200,275		270	4,408,513		$ 0	1,993,942	2,414,201		370	1,562,270		$ 0	928,134	634,494		(358)
Balance (in shares) at Dec. 31, 2020	342,954,023		342,954,023								6,035,205							200							301
Balance at Dec. 31, 2020	$ 14,063,566		$ 1,789,092	8,015,663	4,613,201		(76,411)	(277,979)	5,044,786		$ 60,352	2,810,765	2,173,367		302	4,521,418		$ 0	1,993,942	2,527,167		309	1,542,539		$ 0	928,134	615,018		(613)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	919,468								284,107							382,287							123,452
Dividends on Preferred Stock	(5,600)
Other Comprehensive Income (Loss)	$ 4,546								(38)							166							640
Balance (in shares) at Sep. 30, 2021	343,774,324		343,774,324								6,035,205							200							301
Balance at Sep. 30, 2021	$ 14,411,611		$ 1,789,092	8,050,872	4,906,151		(71,865)	(262,639)	5,114,385		$ 60,352	2,810,765	2,243,004		264	4,679,201		$ 0	2,053,942	2,624,784		475	1,591,031		$ 0	1,088,134	502,870		27
Balance (in shares) at Mar. 31, 2021			343,434,298								6,035,205							200							301
Balance at Mar. 31, 2021	14,233,420		$ 1,789,092	8,016,118	4,772,431		(75,223)	(268,998)	5,071,662		$ 60,352	2,810,765	2,200,275		270	4,408,513		$ 0	1,993,942	2,414,201		370	1,562,270		$ 0	928,134	634,494		(358)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	266,400				266,400				115,556				115,556			110,912				110,912			34,633				34,633
Dividends on Preferred Stock	(1,880)				(1,880)				(1,390)				(1,390)			(490)				(490)
Dividends on Common Stock	(206,893)				(206,893)				(70,100)				(70,100)			(76,800)				(76,800)			(185,200)				(185,200)
Capital Contributions from Eversource Parent																60,000			60,000				160,000			160,000
Long-Term Incentive Plan Activity	6,162			6,162
Issuance of Treasury Shares (in shares)			166,805
Issuance of Treasury Shares	13,799			10,679				3,120
Other Comprehensive Income (Loss)	881						881		3						3	141						141	282						282
Balance (in shares) at Jun. 30, 2021			343,601,103								6,035,205							200							301
Balance at Jun. 30, 2021	14,311,889		$ 1,789,092	8,032,959	4,830,058		(74,342)	(265,878)	5,115,731		$ 60,352	2,810,765	2,244,341		273	4,502,276		$ 0	2,053,942	2,447,823		511	1,571,985		$ 0	1,088,134	483,927		(76)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	285,046				285,046				70,153				70,153			177,451				177,451			44,143				44,143
Dividends on Preferred Stock	(1,880)				(1,880)				(1,390)				(1,390)			(490)				(490)
Dividends on Common Stock	(207,073)				(207,073)				(70,100)				(70,100)										(25,200)				(25,200)
Long-Term Incentive Plan Activity	6,478			6,478
Issuance of Treasury Shares (in shares)			173,221
Issuance of Treasury Shares	14,674			11,435				3,239
Other Comprehensive Income (Loss)	$ 2,477						2,477		(9)						(9)	(36)						(36)	103						103
Balance (in shares) at Sep. 30, 2021	343,774,324		343,774,324								6,035,205							200							301
Balance at Sep. 30, 2021	$ 14,411,611		$ 1,789,092	$ 8,050,872	$ 4,906,151		$ (71,865)	$ (262,639)	$ 5,114,385		$ 60,352	$ 2,810,765	$ 2,243,004		$ 264	$ 4,679,201		$ 0	$ 2,053,942	$ 2,624,784		$ 475	$ 1,591,031		$ 0	$ 1,088,134	$ 502,870		$ 27