Stockholders' Equity - Preferred Stock and Dividend Payments (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions											1 Months Ended				12 Months Ended
	Jun. 30, 2015	Mar. 30, 2015	Dec. 30, 2014	Sep. 30, 2014	Jun. 30, 2014	Apr. 01, 2014	Dec. 30, 2013	Sep. 30, 2013	Jul. 01, 2013	Apr. 01, 2013	Oct. 31, 2015	Feb. 28, 2015	Feb. 28, 2014	Feb. 28, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Preferred Stock
Preferred stock redemption															$ 550.0
Excess of redemption value over carrying value of preferred shares redeemed															8.2
Dividends to preferred stockholders															$ 16.5	$ 33.0	$ 33.0
Preferred stock
Preferred Stock
Excess of redemption value over carrying value of preferred shares redeemed	$ 8.2
Dividends to preferred stockholders	$ 8.3	$ 8.2	$ 8.3	$ 8.2	$ 8.3	$ 8.2	$ 8.3	$ 8.2	$ 8.3	$ 8.2
Common stock
Reconciliation of Outstanding Shares
Outstanding shares at beginning of period															293.9	295.2	293.8
Shares issued															3.5	3.4	5.8
Treasury stock acquired															(6.0)	(4.7)	(4.4)
Outstanding shares at end of period															291.4	293.9	295.2
Common stock share repurchase disclosures
Share repurchase program, maximum authorized amount (in dollars)											$ 150.0	$ 150.0	$ 200.0	$ 150.0
Series A | Preferred stock
Preferred Stock
Preferred stock redemption (in shares)	3.0														3.0
Preferred stock redemption	$ 300.0
Preferred stock dividend rate per annum (as a percent)															5.563%
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
Reconciliation of Outstanding Shares
Outstanding shares at beginning of period															3.0	3.0	3.0
Preferred stock redemption	(3.0)														(3.0)
Outstanding shares at end of period																3.0	3.0
Series B | Preferred stock
Preferred Stock
Preferred stock redemption (in shares)	10.0														10.0
Preferred stock redemption	$ 250.0														$ 0.1
Preferred stock dividend rate per annum (as a percent)															6.518%
Preferred stock dividend (in dollars per share)	$ 0.41	$ 0.41	$ 0.41	$ 0.41	$ 0.41	$ 0.41	$ 0.41	$ 0.41	$ 0.41	$ 0.41
Reconciliation of Outstanding Shares
Outstanding shares at beginning of period															10.0	10.0	10.0
Preferred stock redemption	(10.0)														(10.0)
Outstanding shares at end of period																10.0	10.0