Consolidated Balance Sheets (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Utility Plant, at Original Cost
Nuclear fuel, amortization	132.7	131.1
Common shareholder's equity
Common stock - shares authorized (in shares)	250,000,000	250,000,000
Common stock-shares issued (in shares)	136,406,306	136,113,954
Treasury stock- shares (in shares)	264,567	400,889
Cumulative preferred stock
Cumulative preferred stock par value (in dollars per share)	100.00	100.00
Preferred Stock 3 Point 80 [Member]
Cumulative preferred stock
Cumulative preferred stock, Shares issued (in share)	100,000	100,000
Cumulative preferred stock, Dividend rate (in hundredths)	3.80%	3.80%
Preferred Stock 4 Point 50 [Member]
Cumulative preferred stock
Cumulative preferred stock, Shares issued (in share)	100,000	100,000
Cumulative preferred stock, Dividend rate (in hundredths)	4.50%	4.50%
Preferred Stock 4 Point 20 [Member]
Cumulative preferred stock
Cumulative preferred stock, Shares issued (in share)	70,000	70,000
Cumulative preferred stock, Dividend rate (in hundredths)	4.20%	4.20%
Preferred Stock 4 Point 35 [Member]
Cumulative preferred stock
Cumulative preferred stock, Shares issued (in share)	120,000	120,000
Cumulative preferred stock, Dividend rate (in hundredths)	4.35%	4.35%
Subsidiaries [Member]
Utility Plant, at Original Cost
Nuclear fuel, amortization	132.7	131.1
Common shareholder's equity
Common stock - shares authorized (in shares)	1,000	1,000
Common stock-shares issued (in shares)	1	1