Stockholders' Equity - Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Numerator:
Net (loss) income	$ (56.0)	$ 206.1	$ 122.6	$ 441.6
Denominator:
Weighted average outstanding shares - basic (in shares)	44.9	44.3	44.7	44.8
Effect of dilutive securities:
Warrants (in shares)		2.1	0.1	2.3
Stock options, restricted stock units and employee stock purchase plan (in shares)		0.5	0.9	0.6
Convertible notes (in shares)				0.1
Weighted average shares - diluted (in shares)	44.9	46.9	45.7	47.8
Net (loss) income per common share:
Basic (in dollars per share)	$ (1.25)	$ 4.65	$ 2.74	$ 9.86
Diluted (in dollars per share)	$ (1.25)	$ 4.39	$ 2.68	$ 9.24
Stock options, convertible notes and warrants excluded from calculation (in shares)	4.5	6.5	2.8	5.3