COMMON SHARESHOLDERS' EQUITY AND NONCONTROLLING INTERESTS (NU) (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Shareholders' Equity, Beginning Balance			$ 3,811,176,000
Net Income	91,431,000	101,935,000	285,734,000	262,884,000
Net Income Attributable to Noncontrolling Interests	1,470,000	1,411,000	4,340,000	4,204,000
Other Comprehensive Income/ (Loss)	(9,400,000)	800,000	(11,800,000)	2,500,000
Shareholders' Equity, Ending Balance	3,956,197,000		3,956,197,000
Common Shareholders Equity [Member]
Shareholders' Equity, Beginning Balance	3,915,100,000	3,658,900,000	3,811,200,000	3,577,900,000
Net Income	91,400,000	101,900,000	285,700,000	262,900,000
Dividends on Common Shares	(48,900,000)	(45,400,000)	(146,600,000)	(136,300,000)
Dividends, Preferred Stock	(1,400,000)	(1,400,000)	(4,200,000)	(4,200,000)
Issuance of Common Shares	900,000	1,100,000	5,100,000	6,500,000
Other Transcations, Net	8,500,000	6,800,000	16,800,000	13,400,000
Other Comprehensive Income/ (Loss)	(9,400,000)	800,000	(11,800,000)	2,500,000
Shareholders' Equity, Ending Balance	3,956,200,000	3,722,700,000	3,956,200,000	3,722,700,000
Noncontrolling Interest Member
Shareholders' Equity, Beginning Balance	1,800,000	1,100,000	1,500,000
Contributions To Northern Pass Trans	600,000	300,000	900,000	1,400,000
Net Income Attributable to Noncontrolling Interests	100,000		100,000
Shareholders' Equity, Ending Balance	2,500,000	1,400,000	2,500,000	1,400,000
Total Equity [Member]
Shareholders' Equity, Beginning Balance	3,916,900,000	3,660,000,000	3,812,700,000	3,577,900,000
Net Income	91,400,000	101,900,000	285,700,000	262,900,000
Dividends on Common Shares	(48,900,000)	(45,400,000)	(146,600,000)	(136,300,000)
Dividends, Preferred Stock	(1,400,000)	(1,400,000)	(4,200,000)	(4,200,000)
Issuance of Common Shares	900,000	1,100,000	5,100,000	6,500,000
Contributions To Northern Pass Trans	600,000	300,000	900,000	1,400,000
Other Transcations, Net	8,500,000	6,800,000	16,800,000	13,400,000
Net Income Attributable to Noncontrolling Interests	100,000		100,000
Other Comprehensive Income/ (Loss)	(9,400,000)	800,000	(11,800,000)	2,500,000
Shareholders' Equity, Ending Balance	3,958,700,000	3,724,100,000	3,958,700,000	3,724,100,000
Preferred Stock Member
Shareholders' Equity, Beginning Balance	116,200,000	116,200,000	116,200,000	116,200,000
Dividends, Preferred Stock	(1,400,000)	(1,400,000)	(4,200,000)	(4,200,000)
Net Income Attributable to Noncontrolling Interests	1,400,000	1,400,000	4,200,000	4,200,000
Shareholders' Equity, Ending Balance	$ 116,200,000	$ 116,200,000	$ 116,200,000	$ 116,200,000