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, 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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]																us-gaap:AccountingStandardsUpdate201613Member							us-gaap:AccountingStandardsUpdate201613Member
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		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)