Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Numerator:
Net income	$ 162.7	$ 171.2	$ 363.6	$ 416.0
Denominator:
Weighted average outstanding shares — basic	44.9	44.4	44.8	44.1
Effect of dilutive securities:
Stock options, restricted stock units, and employee stock purchase plan (in shares)	2.7	0.2	2.3	0.3
Weighted average shares - diluted (in shares)	47.6	44.6	47.1	44.4
Net income per common share:
Basic (in dollars per share)	$ 3.62	$ 3.86	$ 8.12	$ 9.43
Diluted (in dollars per share)	$ 3.42	$ 3.84	$ 7.72	$ 9.37
Stock options and restricted stock units excluded from calculation (in shares)	0.0	7.5	0.1	7.4