Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator:
Net (loss) income	$ (494.6)	$ 244.5
Denominator:
Weighted average outstanding shares - basic (in shares)	43.7	43.3
Effect of dilutive securities:
Stock options, restricted stock units and employee stock purchase plan (in shares)		0.6
Weighted average shares - diluted (in shares)	43.7	43.9
Net (loss) income per common share:
Basic (in dollars per share)	$ (11.32)	$ 5.65
Diluted (in dollars per share)	$ (11.32)	$ 5.57
Stock options and restricted stock units excluded from calculation (in shares)	5.3	3.9