Stockholders' Equity - Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net income	$ 276.3	$ 161.8	$ 398.9	$ 603.4
Denominator:
Weighted average outstanding shares - basic (in shares)	43.4	43.2	44.3	44.3
Effect of dilutive securities:
Warrants (in shares)		2.3	0.1	2.3
Stock options, restricted stock units and employee stock purchase plan (in shares)	0.7	0.7	0.8	0.7
Weighted average shares - diluted (in shares)	44.1	46.2	45.2	47.3
Net income per common share:
Basic (in dollars per share)	$ 6.37	$ 3.75	$ 9.00	$ 13.62
Diluted (in dollars per share)	$ 6.27	$ 3.50	$ 8.83	$ 12.76
Stock options, convertible notes and warrants excluded from calculation (in shares)	3.7	5.3	3.1	5.3