Earnings Per Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Numerators
Net income (loss)	$ 29.7		$ 77.0		$ 90.3		$ 59.1		$ (184.3)		$ 82.1		$ 75.4		$ 66.8		$ 256.1	$ 40.0	$ 219.7
Denominators
Weighted average common shares-basic calculation																	76.4	74.9	73.6
Dilutive effect of employee stock options (in shares)																	1.3	1.4	1.9
Weighted average common shares-diluted calculation																	77.7	76.3	75.5
Basic earnings per share (in dollars per share)	$ 0.39	[1]	$ 1.00	[1]	$ 1.18	[1]	$ 0.78	[1]	$ (2.44)	[1]	$ 1.09	[1]	$ 1.01	[1]	$ 0.90	[1]	$ 3.35	$ 0.53	$ 2.99
Diluted earnings per share (in dollars per share)	$ 0.38	[1]	$ 0.99	[1]	$ 1.16	[1]	$ 0.77	[1]	$ (2.44)	[1]	$ 1.07	[1]	$ 0.98	[1]	$ 0.87	[1]	$ 3.30	$ 0.52	$ 2.91
Stock options excluded from computation of diluted earnings per share (in shares)																	0.3	0.4	1.3
Number of warrants not included in diluted earnings per share	10																10

[1] Per share results for the four quarters may differ from full-year per share results, as a separate computation of the weighted average number of shares outstanding is made for each quarter presented.