Stockholders' Equity (Details) - USD ($) shares in Millions, $ in Millions		1 Months Ended			3 Months Ended	6 Months Ended
	Jun. 30, 2015	Feb. 28, 2015	Feb. 28, 2014	Feb. 28, 2013	Jun. 30, 2015	Jun. 30, 2015	Jun. 30, 2014
Preferred Stock
Preferred stock redemption						$ 550.0
Excess of redemption value over carrying value of preferred shares redeemed					$ 8.2	$ 8.2
Preferred stock
Preferred Stock
Excess of redemption value over carrying value of preferred shares redeemed	$ 8.2
Common stock
Reconciliation of Outstanding Shares
Outstanding shares at beginning of period						293.9	295.2
Shares issued						2.3	1.9
Treasury stock acquired						(1.5)	(3.3)
Outstanding shares at end of period	294.7				294.7	294.7	293.8
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
Preferred stock redemption						(3.0)
Outstanding shares at end of period	0.0				0.0	0.0	3.0
Preferred Stock
Preferred stock redemption	$ 250.0
Series B | Preferred stock
Reconciliation of Outstanding Shares
Outstanding shares at beginning of period						10.0	10.0
Preferred stock redemption						(10.0)
Outstanding shares at end of period	0.0				0.0	0.0	10.0
Preferred Stock
Preferred stock redemption	$ 300.0					$ 0.1