Stockholders' Equity - Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Numerator:
Net income	$ 161.8	$ 464.4	$ 603.4	$ 547.0
Denominator:
Weighted average outstanding shares - basic	43.2	45.5	44.3	46.1
Effect of dilutive securities:
Warrants (in shares)	2.3	3.0	2.3	3.1
Stock options, restricted stock units and employee stock purchase plan (in shares)	0.7	1.2	0.7	1.4
Convertible notes (in shares)		0.5		1.1
Weighted average shares - diluted	46.2	50.2	47.3	51.7
Earnings per common share:
Basic (in dollars per share)	$ 3.75	$ 10.20	$ 13.62	$ 11.86
Diluted (in dollars per share)	$ 3.50	$ 9.24	$ 12.76	$ 10.58
Stock options and warrants excluded from calculation	5.3	3.0	5.3	4.1