COMMON SHAREHOLDER EQUITY AND NCI (Details) (USD $) Share data in Thousands, unless otherwise specified	0 Months Ended	3 Months Ended		6 Months Ended
	Apr. 09, 2012	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Shareholders' Equity, Beginning Balance				$ 4,012,670,000
Net Income		46,219,000	78,719,000	146,974,000	194,303,000
Net Income Attributable to Noncontrolling Interests		1,880,000	1,441,000	3,373,000	2,870,000
Other Comprehensive Income/ (Loss)		2,435,000	(4,524,000)	4,299,000	(2,421,000)
Shareholders' Equity, Ending Balance		9,067,639,000		9,067,639,000
Common Stock Issuance, Merger with NSTAR [Abstract]
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares	136,049
Common Shareholders Equity [Member]
Shareholders' Equity, Beginning Balance		4,068,300,000	3,885,300,000
Net Income		46,200,000	78,700,000	147,000,000	194,300,000
Purchase Price Of Nstar		5,038,300,000		5,038,300,000
Other Impacts of Merger With NSTAR		3,400,000		3,400,000
Dividends on Common Shares		(107,600,000)	(48,900,000)	(160,200,000)	(97,700,000)
Dividends, Preferred Stock		(1,900,000)	(1,400,000)	(3,300,000)	(2,800,000)
Issuance of Common Shares		5,200,000		11,400,000	4,200,000
Other Transcations, Net		13,300,000	6,000,000	14,100,000	8,300,000
Net Income Attributable to Noncontrolling Interests				(100,000)
Noncontrolling Interest Member
Shareholders' Equity, Beginning Balance		3,400,000	1,500,000
Other Impacts of Merger With NSTAR		(3,400,000)		(3,400,000)
Contributions To Northern Pass Trans			300,000	300,000	300,000
Net Income Attributable to Noncontrolling Interests				100,000
Total Equity [Member]
Shareholders' Equity, Beginning Balance		4,071,700,000	3,886,800,000
Net Income		46,200,000	78,700,000	147,000,000	194,300,000
Purchase Price Of Nstar		5,038,300,000		5,038,300,000
Dividends on Common Shares		(107,600,000)	(48,900,000)	(160,200,000)	(97,700,000)
Dividends, Preferred Stock		(1,900,000)	(1,400,000)	(3,300,000)	(2,800,000)
Issuance of Common Shares		5,200,000		11,400,000	4,200,000
Contributions To Northern Pass Trans			300,000	300,000	300,000
Other Transcations, Net		13,300,000	6,000,000	14,100,000	8,300,000
Preferred Stock Member
Shareholders' Equity, Beginning Balance		116,200,000	116,200,000
Other Impacts of Merger With NSTAR		39,400,000		39,400,000
Dividends, Preferred Stock		(1,900,000)	(1,400,000)	(3,300,000)	(2,800,000)
Net Income Attributable to Noncontrolling Interests		$ 1,900,000	$ 1,400,000	$ 3,300,000	$ 2,800,000