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, 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)
Long-Term Incentive Plan Activity	(15,320)		(15,320)
Capital Contributions from Eversource Parent							9,000		9,000			92,500		92,500
Other												1			1
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, 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	807,332						298,610					305,666					101,537
Dividends on Preferred Stock	(5,600)
Other Comprehensive Income	6,821						37					325					799
Balance (in shares) at Sep. 30, 2018		316,885,808						6,035,205					200					301
Balance at Sep. 30, 2018	11,408,778	$ 1,669,392	6,234,044	3,882,695	(59,582)	(317,771)	3,921,605	$ 60,352	2,210,765	1,650,182	306	3,695,101	$ 0	1,608,442	2,088,157	(1,498)	1,147,930	$ 0	538,134	612,919	(3,123)
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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	244,647			244,647			99,712			99,712		87,921			87,921		25,779			25,779
Dividends on Common Shares	(160,027)			(160,027)
Dividends on Preferred Stock	(1,880)			(1,880)			(1,390)			(1,390)		(490)			(490)
Long-Term Incentive Plan Activity	4,627		4,627
Capital Contributions from Eversource Parent							91,000		91,000			8,000		8,000			225,000		225,000
Return of Capital																	(530,000)		(530,000)
Other																	(1)			(1)
Other Comprehensive Income	2,142				2,142		9				9	109				109	269				269
Balance (in shares) at Jun. 30, 2018		316,885,808						6,035,205					200					301
Balance at Jun. 30, 2018	11,273,223	$ 1,669,392	6,229,247	3,753,343	(60,988)	(317,771)	3,822,676	$ 60,352	2,210,765	1,551,241	318	3,549,886	$ 0	1,603,442	1,948,052	(1,608)	1,107,003	$ 0	538,134	572,253	(3,384)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	291,260			291,260			100,330			100,330		140,595			140,595		40,666			40,666
Dividends on Common Shares	(160,028)			(160,028)
Dividends on Preferred Stock	(1,880)			(1,880)			(1,389)			(1,389)		(490)			(490)
Long-Term Incentive Plan Activity	4,797		4,797
Capital Contributions from Eversource Parent												5,000		5,000
Other Comprehensive Income	1,406				1,406		(12)				(12)	110				110	261				261
Balance (in shares) at Sep. 30, 2018		316,885,808						6,035,205					200					301
Balance at Sep. 30, 2018	$ 11,408,778	$ 1,669,392	6,234,044	3,882,695	(59,582)	(317,771)	3,921,605	$ 60,352	2,210,765	1,650,182	306	3,695,101	$ 0	1,608,442	2,088,157	(1,498)	1,147,930	$ 0	538,134	612,919	(3,123)
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						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	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	$ 7,983						24					343					881
Balance (in shares) at Sep. 30, 2019	323,733,423	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 Common Shares	(169,857)			(169,857)			(176,400)			(176,400)		(121,200)			(121,200)		(214,000)			(214,000)
Dividends on Preferred Stock	(1,880)			(1,880)			(1,390)			(1,390)		(490)			(490)
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	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 Common Shares	(173,156)			(173,156)			(66,400)			(66,400)							(19,000)			(19,000)
Dividends on Preferred Stock	(1,880)			(1,880)			(1,390)			(1,390)		(490)			(490)
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	$ 1,624				1,624		(1)				(1)	112				112	278				278
Balance (in shares) at Sep. 30, 2019	323,733,423	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)