COMMON SHAREHOLDER EQUIT AND NCI (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Shareholders' Equity, Beginning Balance				$ 3,811,176,000					$ 3,811,176,000
Net Income	114,800,000	91,400,000	78,700,000	115,600,000	131,300,000	101,900,000	73,300,000	87,600,000	400,513,000	394,107,000	335,592,000
Net Income Attributable to Noncontrolling Interests									5,820,000	6,158,000	5,559,000
Other Comprehensive Income/ (Loss)									(27,316,000)	97,000	(6,202,000)
Shareholders' Equity, Ending Balance	4,012,670,000				3,811,176,000				4,012,670,000	3,811,176,000
Common Shareholders Equity [Member]
Net Income									400,513,000	394,107,000	335,592,000
Dividends on Common Shares									(195,595,000)	(181,715,000)	(162,812,000)
Dividends, Preferred Stock									(5,559,000)	(6,101,000)	(5,559,000)
Issuance of Common Shares									5,851,000	7,378,000	389,717,000
Other Transcations, Net									23,900,000	19,900,000
Net Income Attributable to Noncontrolling Interests									(261,000)	(57,000)
Other Comprehensive Income/ (Loss)									(27,316,000)	97,000	(5,474,000)
Noncontrolling Interest Member
Shareholders' Equity, Beginning Balance				1,500,000					1,500,000
Contributions To Northern Pass Trans									1,200,000	1,400,000
Net Income Attributable to Noncontrolling Interests									300,000	100,000
Shareholders' Equity, Ending Balance	3,000,000				1,500,000				3,000,000	1,500,000
Total Equity [Member]
Shareholders' Equity, Beginning Balance				3,812,700,000				3,577,900,000	3,812,700,000	3,577,900,000
Contributions To Northern Pass Trans									1,200,000	1,400,000
Other Transcations, Net									23,900,000	19,900,000	18,700,000
Net Income Attributable to Noncontrolling Interests									0
Shareholders' Equity, Ending Balance	4,015,700,000				3,812,700,000				4,015,700,000	3,812,700,000	3,577,900,000
Preferred Stock Member
Shareholders' Equity, Beginning Balance				116,200,000				116,200,000	116,200,000	116,200,000
Dividends, Preferred Stock									(5,600,000)	(6,100,000)	(5,600,000)
Net Income Attributable to Noncontrolling Interests									5,600,000	6,100,000	5,600,000
Shareholders' Equity, Ending Balance	$ 116,200,000				$ 116,200,000				$ 116,200,000	$ 116,200,000	$ 116,200,000