Stockholders' Equity (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		1 Months Ended					3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 Common stock	Feb. 28, 2013 Common stock	May 31, 2012 Common stock	Mar. 31, 2012 Common stock	Feb. 29, 2012 Common stock	Mar. 31, 2013 Common stock	Mar. 31, 2012 Common stock	Mar. 31, 2013 Series A Preferred stock	Dec. 31, 2012 Series A Preferred stock	Mar. 31, 2012 Series A Preferred stock	Dec. 31, 2011 Series A Preferred stock	Mar. 31, 2013 Series B Preferred stock	Dec. 31, 2012 Series B Preferred stock	Mar. 31, 2012 Series B Preferred stock	Dec. 31, 2011 Series B Preferred stock
Quarterly Common Stock Dividend Paid
Dividends to common stockholders	$ 67.6	$ 54.3	$ 67.6			$ 54.3
Dividend paid on common stock (in dollars per share)			$ 0.23			$ 0.18
Reconciliation of Outstanding Shares
Outstanding shares at beginning of period								293.8	301.1	3.0	3.0	3.0	3.0	10.0	10.0	10.0	10.0
Shares issued								2.4	2.1
Treasury stock acquired								(2.9)	(2.3)
Outstanding shares at end of period			293.3			300.9		293.3	300.9	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)				$ 150.0	$ 200.0		$ 100.0