Stockholders' Equity - Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Numerator:
Net income	$ 178.6	$ 235.5
Denominator:
Weighted average outstanding shares - basic (in shares)	44.5	45.4
Effect of dilutive securities:
Warrants (in shares)	0.3	2.4
Stock options, restricted stock units and employee stock purchase plan (in shares)	1.1	0.8
Convertible notes (in shares)		0.1
Weighted average shares - diluted (in shares)	45.9	48.7
Earnings per common share:
Basic (in dollars per share)	$ 4.01	$ 5.19
Diluted (in dollars per share)	$ 3.89	$ 4.84
Stock options, convertible notes and warrants excluded from calculation (in shares)	1.8	4.1