Stockholders' Equity (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	9 Months Ended	1 Months Ended				3 Months Ended			9 Months Ended
	Sep. 30, 2012	May 31, 2012 Common stock	Feb. 29, 2012 Common stock	Nov. 30, 2011 Common stock	Aug. 31, 2011 Common stock	Sep. 30, 2012 Common stock	Jun. 30, 2012 Common stock	Mar. 31, 2012 Common stock	Sep. 30, 2012 Common stock	Sep. 30, 2011 Common stock	Sep. 30, 2012 Series A Preferred stock	Dec. 31, 2011 Series A Preferred stock	Sep. 30, 2011 Series A Preferred stock	Dec. 31, 2010 Series A Preferred stock	Sep. 30, 2012 Series B Preferred stock	Dec. 31, 2011 Series B Preferred stock	Sep. 30, 2011 Series B Preferred stock	Dec. 31, 2010 Series B Preferred stock
Quarterly Common Stock Dividend Paid
Dividends to common stockholders	$ 169.6					$ 61.6	$ 53.7	$ 54.3
Dividend paid on common stock (in dollars per share)						$ 0.21	$ 0.18	$ 0.18
Reconciliation of Outstanding Shares
Outstanding shares at beginning of period								301.1	301.1	320.4	3.0	3.0	3.0	3.0	10.0	10.0	10.0	10.0
Shares issued									3.0	1.7
Treasury stock acquired									(10.5)	(17.0)
Outstanding shares at end of period						293.6			293.6	305.1	3.0	3.0	3.0	3.0	10.0	10.0	10.0	10.0
Common stock share repurchase disclosures
Share repurchase program, maximum authorized amount (in dollars)		$ 200.0	$ 100.0	$ 100.0	$ 200.0