CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDERS EQUITY - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Treasury Stock [Member]	Common Shareholders Equity [Member]	The Connecticut Light And Power Company [Member]	The Connecticut Light And Power Company [Member] Common Stock [Member]	The Connecticut Light And Power Company [Member] Additional Paid-in Capital [Member]	The Connecticut Light And Power Company [Member] Retained Earnings [Member]	The Connecticut Light And Power Company [Member] Accumulated Other Comprehensive Income (Loss) [Member]	The Connecticut Light And Power Company [Member] Common Shareholders Equity [Member]	NSTAR Electric Company [Member]	NSTAR Electric Company [Member] Common Stock [Member]	NSTAR Electric Company [Member] Additional Paid-in Capital [Member]	NSTAR Electric Company [Member] Retained Earnings [Member]	NSTAR Electric Company [Member] Accumulated Other Comprehensive Income (Loss) [Member]	NSTAR Electric Company [Member] Stockholders Equity Total Member	NSTAR Electric Company [Member] Common Shareholders Equity [Member]	Public Service Company Of New Hampshire [Member]	Public Service Company Of New Hampshire [Member] Common Stock [Member]	Public Service Company Of New Hampshire [Member] Additional Paid-in Capital [Member]	Public Service Company Of New Hampshire [Member] Retained Earnings [Member]	Public Service Company Of New Hampshire [Member] Accumulated Other Comprehensive Income (Loss) [Member]	Public Service Company Of New Hampshire [Member] Common Shareholders Equity [Member]	Western Massachusetts Electric Company [Member]	Western Massachusetts Electric Company [Member] Common Stock [Member]	Western Massachusetts Electric Company [Member] Additional Paid-in Capital [Member]	Western Massachusetts Electric Company [Member] Retained Earnings [Member]	Western Massachusetts Electric Company [Member] Accumulated Other Comprehensive Income (Loss) [Member]	Western Massachusetts Electric Company [Member] Common Shareholders Equity [Member]
Common Stock, Shares, Outstanding		314,053,634							6,035,205						100							301						434,653
Common Stock, Value, Outstanding		$ 1,662,547	$ 6,183,267	$ 1,802,714	$ (72,854)	$ (338,624)	$ 9,237,050		$ 60,352	$ 1,640,149	$ 839,628	$ (1,800)	$ 2,538,329		$ 0	$ 992,625	$ 1,210,405			$ 2,203,030		$ 0	$ 701,052	$ 395,118	$ (9,655)	$ 1,086,515		$ 10,866	$ 390,412	$ 160,577	$ (3,846)	$ 558,009
Net Income	$ 793,689							$ 279,412			279,412		279,412	$ 268,546			268,546			268,546	$ 111,397			111,397		111,397	$ 60,438			60,438		60,438
Dividends on Common Shares				(462,741)			(462,741)				(151,999)		(151,999)				(56,000)			(56,000)				(68,000)		(68,000)				(40,001)		(40,001)
Issuance of Common Shares		$ 2,804	8,274				11,078
Stock Issued During Period, Shares, New Issues		560,848
Dividends on Preferred Stock				(7,682)			(7,682)				(5,559)		(5,559)				(2,123)			(2,123)
Stock Issued During Period, Value, Employee Stock Ownership Plan										1,847			1,847										859			859			331			331
Long-Term Incentive Plan Activity			(10,748)				(10,748)
Other Comprehensive Income (Loss), Net of Tax	26,823				26,823		26,823	413				413									1,105				1,105	1,105	329				329	329
Stock Issued During Period, Value, Treasury Stock Reissued			17,381			12,087	29,468
Stock Issued During Period, Shares, Treasury Stock Reissued		659,077
Payments of Stock Issuance Costs										51			51
Net Income Attributable to Noncontrolling Interests	7,682
Capital Contributions from NU Parent								40,000		40,000			40,000								0
OtherChangesInShareholdersEquity			(5,409)				(5,409)
Common Stock, Shares, Outstanding		315,273,559							6,035,205						100							301						434,653
Common Stock, Value, Outstanding		$ 1,665,351	6,192,765	2,125,980	(46,031)	(326,537)	9,611,528		$ 60,352	1,682,047	961,482	(1,387)	2,702,494		$ 0	992,625	1,420,828			2,413,453		$ 0	701,911	438,515	(8,550)	1,131,876		$ 10,866	390,743	181,014	(3,517)	579,106
Net Income	827,065			827,065			827,065	287,754			287,754		287,754	303,088			303,088			303,088	113,944			113,944		113,944	57,819			57,819		57,819
Dividends on Common Shares				(496,524)			(496,524)				(171,200)		(171,200)				(253,000)			(253,000)				(66,000)		(66,000)				(59,999)		(59,999)
Issuance of Common Shares		$ 1,445	37,817				6,609
Stock Issued During Period, Shares, New Issues		288,941,000
Dividends on Preferred Stock				(7,519)			(7,519)				(5,559)		(5,559)				(1,961)			(1,961)
Stock Issued During Period, Value, Employee Stock Ownership Plan										2,771			2,771										1,329			1,329			$ 513			513
Long-Term Incentive Plan Activity			$ 9,657				(9,569)
Other Comprehensive Income (Loss), Net of Tax	(27,978)				(27,978)		(27,978)	456				456	456	442							1,181				1,181	1,181	341
Stock Issued During Period, Value, Treasury Stock Reissued						26,070	63,887
Stock Issued During Period, Shares, Treasury Stock Reissued		1,420,837,000
Payments of Stock Issuance Costs										51			51
Net Income Attributable to Noncontrolling Interests	$ 7,519
Capital Contributions from NU Parent								120,000		120,000			120,000								45,000											341
SharesIssuedInConnectionWithNstarMerger
SharesIssuedValueInConectionWithNstarMerger
OtherChangesInShareholdersEquity							9,316									1,505
Common Stock, Shares, Outstanding	316,983,337	316,983,337							6,035,205						100							301						434,653
Common Stock, Value, Outstanding		$ 1,666,796	$ 6,235,834	2,448,661	(74,009)	(300,467)	9,976,815		$ 60,352	1,804,869	1,072,477	(931)	2,936,767		$ 0	994,130	1,468,955			2,463,527		$ 0	748,240	486,459	(7,369)	1,227,330		$ 10,866	$ 391,256	178,834	(3,176)	577,780
OtherEquityImpactsOfMergerWithNstar
Net Income	$ 886,004			886,004			886,004	299,360			299,360		299,360	344,542			344,542			344,542	114,442			114,442		114,442	56,506			56,506		56,506
Dividends on Common Shares				(529,791)			(529,791)				(196,000)		(196,000)				(197,999)			(197,999)				(106,000)		(106,000)				(37,200)		(37,200)
Issuance of Common Shares		$ 2,517	$ 6,951				9,468
Stock Issued During Period, Shares, New Issues		503,443
Dividends on Preferred Stock				(7,519)			(7,519)				(5,559)		(5,559)				(1,960)			(1,960)
Stock Issued During Period, Value, Employee Stock Ownership Plan										743			743										394			394			142			142
Long-Term Incentive Plan Activity			(6,140)				(6,140)
Other Comprehensive Income (Loss), Net of Tax	7,165				7,165		7,165	355				355	355	$ 103				$ 103	$ 103		1,008				1,008	1,008	$ 354				354	354
Stock Issued During Period, Shares, Treasury Stock Reissued		(295,531)
Payments of Stock Issuance Costs										51			51
Net Income Attributable to Noncontrolling Interests	$ 7,519
Capital Contributions from NU Parent								$ 105,000		105,000			105,000								$ 0
OtherChangesInShareholdersEquity			3,653				3,653										1,248
IncreaseInTreasuryShares			22,070			(9,510)	12,560
Common Stock, Shares, Outstanding	317,191,249	317,191,249							6,035,205						100							301						434,653
Common Stock, Value, Outstanding		$ 1,669,313	$ 6,262,368	$ 2,797,355	$ (66,844)	$ (309,977)	$ 10,352,215		$ 60,352	$ 1,910,663	$ 1,170,278	$ (576)	$ 3,140,717		$ 0	$ 995,378	$ 1,613,538			$ 2,609,461		$ 0	$ 748,634	$ 494,901	$ (6,361)	$ 1,237,174		$ 10,866	$ 391,398	$ 198,140	$ (2,822)	$ 597,582