COMMON SHAREHOLDER EQUIT AND NCI (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Shareholders' Equity, Beginning Balance			$ 9,237,050,000
Net Income	211,380,000	209,508,000	614,413,000	356,482,000
Net Income Attributable to Noncontrolling Interests	1,879,000	1,880,000	5,803,000	5,253,000
Other Comprehensive Income (Loss), Net of Tax	2,082,000	2,178,000	5,467,000	6,477,000
Shareholders' Equity, Ending Balance	9,517,453,000		9,517,453,000
Common Shareholders Equity [Member]
Shareholders' Equity, Beginning Balance	9,406,600,000	9,067,600,000	9,237,100,000	4,012,700,000
Net Income	211,400,000	209,500,000	614,400,000	356,500,000
Dividends on Common Shares	(114,900,000)	(107,600,000)	(346,900,000)	(267,800,000)
Dividends, Preferred Stock	(1,900,000)	(1,900,000)	(5,800,000)	(5,100,000)
Issuance of Common Shares	1,400,000	800,000	10,200,000	12,200,000
Other Transcations, Net	12,800,000	6,300,000	3,000,000	20,300,000
Net Income Attributable to Noncontrolling Interests				(100,000)
Other Comprehensive Income (Loss), Net of Tax	2,100,000	2,200,000	5,500,000	6,500,000
Shareholders' Equity, Ending Balance	9,517,500,000	9,176,900,000	9,517,500,000	9,176,900,000
Purchase Price of NSTAR				5,038,300,000
Other Impacts of Merger with NSTAR				3,400,000
Noncontrolling Interest Member
Shareholders' Equity, Beginning Balance				3,000,000
Contributions To Northern Pass Trans				300,000
Net Income Attributable to Noncontrolling Interests				100,000
Other Impacts of Merger with NSTAR				(3,400,000)
Total Equity [Member]
Shareholders' Equity, Beginning Balance		9,067,600,000		4,015,700,000
Net Income		209,500,000		356,500,000
Dividends on Common Shares		(107,600,000)		(267,800,000)
Dividends, Preferred Stock		(1,900,000)		(5,100,000)
Issuance of Common Shares		800,000		12,200,000
Contributions To Northern Pass Trans				300,000
Other Transcations, Net		6,300,000		20,300,000
Other Comprehensive Income (Loss), Net of Tax		2,200,000		6,500,000
Shareholders' Equity, Ending Balance		9,176,900,000		9,176,900,000
Purchase Price of NSTAR				5,038,300,000
Preferred Stock Member
Shareholders' Equity, Beginning Balance	155,600,000	155,600,000	155,600,000	116,200,000
Dividends, Preferred Stock	(1,900,000)	(1,900,000)	(5,800,000)	(5,100,000)
Net Income Attributable to Noncontrolling Interests	1,900,000	1,900,000	5,800,000	5,100,000
Shareholders' Equity, Ending Balance	155,600,000	155,600,000	155,600,000	155,600,000
Other Impacts of Merger with NSTAR				$ 39,400,000