Net Income Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Earnings Per Share [Abstract]
Net income	$ 187.3	$ 151.2	$ 91.2	$ 74.3	$ 338.5	$ 165.5
Weighted average common shares outstanding (in shares)	74,000,000.0		73,800,000		74,000,000.0	73,800,000
Dilutive effect of equity awards, based on the treasury stock method (in shares)	1,700,000		1,700,000		1,800,000	1,700,000
Weighted average shares assuming dilution (in shares)	75,700,000		75,500,000		75,800,000	75,500,000
Antidilutive options excluded from computation of diluted net income per share (in shares)	100,000		200,000		100,000	200,000
Stock repurchase program, shares authorized (in shares)							631,000
Share purchased under share repurchase program (in shares)	0				479,000
Stock purchase program, cost of shares purchased		$ 137.1		$ 115.5	$ 137.1
Stock repurchase program, average price per share (in dollars per share)					$ 286.23