Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator:
Net income (loss)	$ 132.4	$ 106.5	$ (157.1)	$ 523.9
Denominator:
Weighted average outstanding shares - basic (in shares)	43.9	43.6	43.8	43.5
Effect of dilutive securities:
Stock options, restricted stock units and employee stock purchase plan (in shares)	0.1	0.4		0.5
Weighted average shares - diluted (in shares)	44.0	44.0	43.8	44.0
Net income (loss) per common share:
Basic (in dollars per share)	$ 3.02	$ 2.44	$ (3.59)	$ 12.04
Diluted (in dollars per share)	$ 3.01	$ 2.42	$ (3.59)	$ 11.91
Stock options and restricted stock units excluded from calculation (in shares)	8.0	4.5	7.1	4.6