COMMON SHAREHOLDER EQUITY AND NCI (Details) (USD $) Share data in Thousands, unless otherwise specified	0 Months Ended	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended			3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended
	Apr. 09, 2012	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Common Shareholders Equity [Member]	Sep. 30, 2011 Common Shareholders Equity [Member]	Sep. 30, 2012 Common Shareholders Equity [Member]	Sep. 30, 2011 Common Shareholders Equity [Member]	Sep. 30, 2011 Noncontrolling Interest Member	Sep. 30, 2012 Noncontrolling Interest Member	Sep. 30, 2011 Noncontrolling Interest Member	Jun. 30, 2012 Noncontrolling Interest Member	Sep. 30, 2012 Total Equity [Member]	Sep. 30, 2011 Total Equity [Member]	Sep. 30, 2012 Total Equity [Member]	Sep. 30, 2011 Total Equity [Member]	Sep. 30, 2012 Preferred Stock Member	Sep. 30, 2011 Preferred Stock Member	Sep. 30, 2012 Preferred Stock Member	Sep. 30, 2011 Preferred Stock Member
Shareholders' Equity, Beginning Balance				$ 4,012,670,000		$ 9,067,600,000	$ 3,915,100,000			$ 1,800,000			$ 0	$ 9,067,600,000	$ 3,916,900,000			$ 155,600,000	$ 116,200,000
Net Income		209,508,000	91,431,000	356,482,000	285,734,000	209,500,000	91,400,000	356,500,000	285,700,000					209,500,000	91,400,000	356,500,000	285,700,000
Purchase Price Of Nstar								5,038,300,000								5,038,300,000
Other Impacts of Merger With NSTAR								3,400,000			(3,400,000)									39,400,000
Dividends on Common Shares						(107,600,000)	(48,900,000)	(267,800,000)	(146,600,000)					(107,600,000)	(48,900,000)	(267,800,000)	(146,600,000)
Dividends, Preferred Stock						(1,900,000)	(1,400,000)	(5,100,000)	(4,200,000)					(1,900,000)	(1,400,000)	(5,100,000)	(4,200,000)	(1,900,000)	(1,400,000)	(5,100,000)	(4,200,000)
Issuance of Common Shares						800,000	900,000	12,200,000	5,100,000					800,000	900,000	12,200,000	5,100,000
Contributions To Northern Pass Trans					900,000					600,000	300,000				600,000	300,000	900,000
Other Transcations, Net						6,300,000	8,500,000	20,300,000	16,800,000					6,300,000	8,500,000	20,300,000	16,800,000
Net Income Attributable to Noncontrolling Interests		1,880,000	1,470,000	5,253,000	4,340,000			(100,000)		100,000	100,000	100,000			100,000		100,000	1,900,000	1,400,000	5,100,000	4,200,000
Other Comprehensive Income/ (Loss)		2,178,000	(9,423,000)	6,477,000	(11,844,000)
Shareholders' Equity, Ending Balance		$ 9,176,866,000		$ 9,176,866,000									$ 0
Common Stock Issuance, Merger with NSTAR [Abstract]
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares	136,049