Stockholders' Equity (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended			3 Months Ended
	Feb. 28, 2015	Feb. 28, 2014	Feb. 28, 2013	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013
Common stock
Reconciliation of Outstanding Shares
Outstanding shares at beginning of period				293.9	295.2
Shares issued				2.0	1.6
Treasury stock acquired				(1.5)	(2.0)
Outstanding shares at end of period				294.4	294.8
Common stock share repurchase disclosures
Share repurchase program, maximum authorized amount (in dollars)	$ 150.0	$ 200.0	$ 150.0
Series A | Preferred stock
Reconciliation of Outstanding Shares
Outstanding shares at beginning of period						3.0	3.0
Outstanding shares at end of period				3.0	3.0	3.0	3.0
Series B | Preferred stock
Reconciliation of Outstanding Shares
Outstanding shares at beginning of period						10.0	10.0
Outstanding shares at end of period				10.0	10.0	10.0	10.0