Stockholders' Equity (Details 2) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended			1 Months Ended						12 Months Ended			1 Months Ended												12 Months Ended		1 Months Ended												12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Common stock	Nov. 30, 2011 Common stock	Aug. 31, 2011 Common stock	Dec. 31, 2010 Common stock	Dec. 31, 2009 Common stock	May 31, 2009 Common stock	Dec. 31, 2011 Common stock	Dec. 31, 2010 Common stock	Dec. 31, 2009 Common stock	Dec. 31, 2011 Series A Preferred stock	Sep. 30, 2011 Series A Preferred stock	Jun. 30, 2011 Series A Preferred stock	Mar. 31, 2011 Series A Preferred stock	Dec. 31, 2010 Series A Preferred stock	Sep. 30, 2010 Series A Preferred stock	Jun. 30, 2010 Series A Preferred stock	Mar. 31, 2010 Series A Preferred stock	Dec. 31, 2009 Series A Preferred stock	Sep. 30, 2009 Series A Preferred stock	Jun. 30, 2009 Series A Preferred stock	Mar. 31, 2009 Series A Preferred stock	Dec. 31, 2011 Series A Preferred stock	Dec. 31, 2008 Series A Preferred stock	Dec. 31, 2011 Series B Preferred stock	Sep. 30, 2011 Series B Preferred stock	Jun. 30, 2011 Series B Preferred stock	Mar. 31, 2011 Series B Preferred stock	Dec. 31, 2010 Series B Preferred stock	Sep. 30, 2010 Series B Preferred stock	Jun. 30, 2010 Series B Preferred stock	Mar. 31, 2010 Series B Preferred stock	Dec. 31, 2009 Series B Preferred stock	Sep. 30, 2009 Series B Preferred stock	Jun. 30, 2009 Series B Preferred stock	Mar. 31, 2009 Series B Preferred stock	Dec. 31, 2011 Series B Preferred stock	Dec. 31, 2008 Series B Preferred stock
Preferred Stock
Preferred stock dividend rate per annum (as a percent)																									5.563%														6.518%
Preferred stock redemption price (in dollars per share)													$ 100												$ 100		$ 25												$ 25
Preferred stock aggregate redemption amount													$ 300.0												$ 300.0		$ 250.0												$ 250.0
Preferred stock dividend (in dollars per share)													$ 1.39	$ 1.39	$ 1.39	$ 1.39	$ 1.39	$ 1.39	$ 1.39	$ 1.39	$ 1.39	$ 1.39	$ 1.39	$ 1.39			$ 0.41	$ 0.41	$ 0.41	$ 0.41	$ 0.41	$ 0.41	$ 0.41	$ 0.41	$ 0.41	$ 0.41	$ 0.41	$ 0.41
Annual Common Stock Dividend Paid
Dividends to common stockholders	213.7	176.2	159.5	213.7			176.2	159.5
Annual dividend paid on common stock (in dollars per share)				$ 0.70			$ 0.55	$ 0.50
Reconciliation of Outstanding Shares
Outstanding shares at beginning of period										320.4	319.0	259.3													3.0	3.0													10.0	10.0
Shares issued										1.8	1.5	60.0
Treasury stock acquired										(21.1)	(0.1)	(0.3)
Outstanding shares at end of period				301.1			320.4	319.0		301.1	320.4	319.0	3.0				3.0				3.0				3.0	3.0	10.0				10.0				10.0				10.0	10.0
Common Stock Issuance
Shares of common stock issued in May 2009 (in shares)									58.2
Price per share of common stock issued in May 2009 (in dollars per share)									$ 19.75
Net proceeds from issuance of common stock in May 2009									1,109.1
Common stock share repurchase disclosures
Share repurchase program, maximum authorized amount (in dollars)					$ 100.0	$ 200.0