Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerators
Net income (loss)	$ 20.3	$ 13.8	$ 57.4	$ 52.7	$ 15.5	$ 74.5	$ 136.2	$ 50.1	$ 144.2	$ 276.3	$ 256.1
Denominators
Weighted average common shares-basic calculation									72,000,000	75,300,000	76,400,000
Dilutive effect of employee stock options (in shares)									600,000	900,000	1,300,000
Weighted average common shares-diluted calculation									72,600,000	76,200,000	77,700,000
Basic earnings per share (in dollars per share)	$ 0.29	$ 0.19	$ 0.79	$ 0.72	$ 0.21	$ 1.00	$ 1.79	$ 0.65	$ 2.00	$ 3.67	$ 3.35
Diluted earnings per share (in dollars per share)	$ 0.29	$ 0.19	$ 0.78	$ 0.72	$ 0.21	$ 0.99	$ 1.77	$ 0.64	$ 1.99	$ 3.63	$ 3.30
Stock options excluded from computation of diluted earnings per share (in shares)									400,000	300,000	300,000
Share Repurchase Program [Line Items]
Stock Repurchase Program, Number of Shares Authorized to be Repurchased	500,000,000				500,000,000				500,000,000	500,000,000
Stock Repurchased During Period, Shares									3,123,716	4,936,497
Payments for Repurchase of Common Stock									$ 301.5	$ 469.5	$ 0.0
Minimum
Share Repurchase Program [Line Items]
Stock Repurchase Program, Period in Force									12 months
Maximum
Share Repurchase Program [Line Items]
Stock Repurchase Program, Period in Force									24 months