COMMON SHAREHOLDER EQUIT AND NCI (Details) (USD $)	3 Months Ended				6 Months Ended
	Jun. 30, 2013	Mar. 31, 2013	Jun. 30, 2012	Mar. 31, 2012	Jun. 30, 2013	Jun. 30, 2012
Shareholders' Equity, Beginning Balance		$ 9,237,050,000			$ 9,237,050,000
Net Income	173,064,000		46,219,000		403,032,000	146,974,000
Dividends on Common Shares					(232,100,000)	(160,200,000)
Dividends, Preferred Stock					(3,900,000)	(3,300,000)
Issuance of Common Shares					8,800,000	11,400,000
Contributions To Northern Pass Trans						300,000
Other Transcations, Net					(9,700,000)	14,100,000
Net Income Attributable to Noncontrolling Interests	2,043,000		1,880,000		3,922,000	3,373,000
Other Comprehensive Income (Loss), Net of Tax	1,429,000		2,435,000		3,385,000	4,299,000
Shareholders' Equity, Ending Balance	9,406,612,000				9,406,612,000
Purchase Price of NSTAR			5,038,300,000			5,038,300,000
Other Impacts of Merger with NSTAR						3,400,000
Common Shareholders Equity [Member]
Shareholders' Equity, Beginning Balance		9,345,200,000		4,068,300,000	9,345,200,000	4,068,300,000
Net Income	173,100,000		46,200,000		403,000,000	147,000,000
Dividends on Common Shares	(115,600,000)		(107,600,000)
Dividends, Preferred Stock	(2,000,000)		(1,900,000)
Issuance of Common Shares	300,000		5,200,000
Other Transcations, Net	4,200,000		13,300,000
Net Income Attributable to Noncontrolling Interests			0			(100,000)
Other Comprehensive Income (Loss), Net of Tax	1,400,000		2,400,000		3,400,000	4,300,000
Shareholders' Equity, Ending Balance	9,406,600,000		9,067,600,000		9,406,600,000	9,067,600,000
Purchase Price of NSTAR			5,038,300,000			5,038,300,000
Other Impacts of Merger with NSTAR		9,237,100,000	3,400,000	4,012,700,000
Noncontrolling Interest Member
Shareholders' Equity, Beginning Balance				3,400,000		3,400,000
Dividends, Preferred Stock					(3,900,000)
Net Income Attributable to Noncontrolling Interests			0		3,900,000	100,000
Shareholders' Equity, Ending Balance			0			0
Other Impacts of Merger with NSTAR			(3,400,000)	3,000,000		(3,400,000)
Total Equity [Member]
Shareholders' Equity, Beginning Balance				4,071,700,000		4,071,700,000
Net Income			46,200,000			147,000,000
Dividends on Common Shares			(107,600,000)
Dividends, Preferred Stock			(1,900,000)
Issuance of Common Shares			5,200,000
Other Transcations, Net			13,300,000
Other Comprehensive Income (Loss), Net of Tax			2,400,000			4,300,000
Shareholders' Equity, Ending Balance			9,067,600,000			9,067,600,000
Other Impacts of Merger with NSTAR				4,015,700,000
Preferred Stock Member
Shareholders' Equity, Beginning Balance		155,600,000		116,200,000	155,600,000	116,200,000
Dividends, Preferred Stock	(2,000,000)		(1,900,000)
Net Income Attributable to Noncontrolling Interests	2,000,000		1,900,000			3,300,000
Shareholders' Equity, Ending Balance	155,600,000		155,600,000		155,600,000	155,600,000
Other Impacts of Merger with NSTAR		$ 155,600,000	$ 39,400,000	$ 116,200,000		$ 39,400,000