Net Income Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Net income	$ 98.4	$ 82.3	$ 91.2	$ 74.3	$ 63.9	$ 56.3	$ 66.1	$ 55.4	$ 346.2	$ 241.7	$ 206.9
Weighted average common shares outstanding (in shares)									73,900,000	74,000,000.0	73,900,000
Dilutive effect of equity awards, based on the treasury stock method (in shares)									1,900,000	1,400,000	1,500,000
Weighted average shares assuming dilution (in shares)									75,800,000	75,400,000	75,400,000
Antidilutive options excluded from computation of diluted net income per share (in shares)									0.0	100,000	400,000
Stock repurchase program, shares authorized (in shares)	631,000				848,000				631,000	848,000
Shares purchased under share repurchase program (in shares)	0								761,500
Shares purchased under share repurchase program									$ 115.5	$ 83.1	$ 70.8
Shares purchased under share repurchase program, average cost per share (in dollars per share)									$ 151.65