CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Shares	Capital Surplus, Paid In	Retained Earnings	Accumulated Other Comprehensive 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 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 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 Loss
Beginning Balance (in shares) at Dec. 31, 2014		316,983,337						6,035,205					200					301
Beginning balance at Dec. 31, 2014	$ 9,976,815	$ 1,666,796	$ 6,235,834	$ 2,448,661	$ (74,009)	$ (300,467)	$ 2,936,767	$ 60,352	$ 1,804,869	$ 1,072,477	$ (931)	$ 3,041,308	$ 0	$ 1,396,252	$ 1,647,790	$ (2,734)	$ 1,227,330	$ 0	$ 748,240	$ 486,459	$ (7,369)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	886,004			886,004			299,360			299,360		401,048			401,048		114,442			114,442
Dividends on Common Stock	(529,791)			(529,791)			(196,000)			(196,000)		(235,200)			(235,200)		(106,000)			(106,000)
Dividends on Preferred Stock	(7,519)			(7,519)			(5,559)			(5,559)		(1,960)			(1,960)
Allocation of Benefits - ESOP							743		743								394		394
Capital Stock Expenses, Net							51		51
Capital Contributions from Eversource Parent							105,000		105,000
Issuance of Common Shares (in shares)		503,443
Issuance of Common Shares	9,468	$ 2,517	6,951
Long-Term Incentive Plan Activity	(6,140)		(6,140)
Increase in Treasury Shares (in shares)		(295,531)
Increase in Treasury Shares	12,560		22,070			(9,510)
Other Changes in Shareholders' Equity	3,653		3,653									1,390		1,390
Other Comprehensive Income	7,165				7,165		355				355	458				458	1,008				1,008
Ending Balance (in shares) at Dec. 31, 2015		317,191,249						6,035,205					200					301
Ending balance at Dec. 31, 2015	10,352,215	$ 1,669,313	6,262,368	2,797,355	(66,844)	(309,977)	3,140,717	$ 60,352	1,910,663	1,170,278	(576)	3,207,044	$ 0	1,397,642	1,811,678	(2,276)	1,237,174	$ 0	748,634	494,901	(6,361)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	949,821			949,821			334,254			334,254		350,777			350,777		131,985			131,985
Dividends on Common Stock	(564,486)			(564,486)			(199,599)			(199,599)		(316,300)			(316,300)		(77,600)			(77,600)
Dividends on Preferred Stock	(7,519)			(7,519)			(5,559)			(5,559)		(1,960)			(1,960)
Capital Stock Expenses, Net							51		51
Capital Contributions from Eversource Parent							200,000		200,000			103,000		103,000			94,500		94,500
Issuance of Common Shares (in shares)		15,787
Issuance of Common Shares	(5,560)	$ 79	(5,639)
Long-Term Incentive Plan Activity	(6,056)		(6,056)
Increase in Treasury Shares (in shares)		(321,228)
Increase in Treasury Shares	(7,794)					(7,794)
Other Changes in Shareholders' Equity	(449)		(449)
Other Comprehensive Income	1,562				1,562		523				523	282				282	1,298				1,298
Ending Balance (in shares) at Dec. 31, 2016		316,885,808						6,035,205					200					301
Ending balance at Dec. 31, 2016	10,711,734	$ 1,669,392	6,250,224	3,175,171	(65,282)	(317,771)	3,470,387	$ 60,352	2,110,714	1,299,374	(53)	3,342,843	$ 0	1,500,642	1,844,195	(1,994)	1,387,357	$ 0	843,134	549,286	(5,063)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	995,515			995,515			376,726					374,726					135,996
Dividends on Common Stock	(602,083)			(602,083)			(254,800)			(254,800)		(272,000)			(272,000)		(173,900)			(173,900)
Dividends on Preferred Stock	(7,519)			(7,519)			(5,559)			(5,559)		(1,960)			(1,960)
Capital Stock Expenses, Net							51		51
Capital Contributions from Eversource Parent												2,300		2,300
Long-Term Incentive Plan Activity	(10,834)		(10,834)
Other Changes in Shareholders' Equity	550		550
Other Comprehensive Income	(1,121)				(1,121)		322				322	171				171	1,141				1,141
Ending Balance (in shares) at Dec. 31, 2017		316,885,808						6,035,205					200					301
Ending 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)