Common Shareholder's Equity (Details) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Authorized, reserved common and preferred shares [Abstract]
Common stock, shares authorized for issue (in shares)	480.0	480.0
Preferred stock , shares authorized for issue (in shares)	20.0	20.0
Number of shares reserved for issuance under share-based compensation plans, benefit plans, and dividend reinvestment plan (in shares)	6.8	5.5
Authorized shares of preferred stock available for issuance (in shares)	20.0	20.0
Authorized shares of common stock available for issuance (in shares)	391.3	392.8