COMMON SHAREHOLDER EQUITY AND NCI (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Shareholders' Equity, Beginning Balance	$ 4,012,670,000
Net Income	100,755,000	115,584,000
Net Income Attributable to Noncontrolling Interests	1,493,000	1,429,000
Other Comprehensive Income/ (Loss)	1,864,000	2,103,000
Shareholders' Equity, Ending Balance	4,068,301,000
Common Shareholders Equity [Member]
Shareholders' Equity, Beginning Balance	4,012,700,000	3,811,200,000
Net Income	100,800,000	115,600,000
Dividends on Common Shares	(52,600,000)	(48,800,000)
Dividends, Preferred Stock	(1,400,000)	(1,400,000)
Issuance of Common Shares	6,200,000	4,200,000
Contributions To Northern Pass Trans	0
Other Transcations, Net	800,000	2,400,000
Net Income Attributable to Noncontrolling Interests	(100,000)
Other Comprehensive Income/ (Loss)	1,900,000	2,100,000
Shareholders' Equity, Ending Balance	4,068,300,000	3,885,300,000
Noncontrolling Interest Member
Shareholders' Equity, Beginning Balance	3,000,000	1,500,000
Contributions To Northern Pass Trans	300,000	0
Net Income Attributable to Noncontrolling Interests	100,000	0
Shareholders' Equity, Ending Balance	3,400,000	1,500,000
Total Equity [Member]
Shareholders' Equity, Beginning Balance	4,015,700,000	3,812,700,000
Net Income	100,800,000	115,600,000
Dividends on Common Shares	(52,600,000)	(48,800,000)
Dividends, Preferred Stock	(1,400,000)	(1,400,000)
Issuance of Common Shares	6,200,000	4,200,000
Contributions To Northern Pass Trans	300,000
Other Transcations, Net	800,000	2,400,000
Other Comprehensive Income/ (Loss)	1,900,000	2,100,000
Shareholders' Equity, Ending Balance	4,071,700,000	3,886,800,000
Preferred Stock Member
Shareholders' Equity, Beginning Balance	116,200,000	116,200,000
Dividends, Preferred Stock	(1,400,000)	(1,400,000)
Net Income Attributable to Noncontrolling Interests	1,400,000	1,400,000
Shareholders' Equity, Ending Balance	$ 116,200,000	$ 116,200,000