Net Income Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Earnings Per Share [Abstract]
Net income	$ 82.3	$ 91.2	$ 74.3	$ 56.3	$ 66.1	$ 55.4	$ 247.8	$ 177.8
Weighted average common shares outstanding (in shares)	73,900,000			74,000,000.0			73,900,000	73,900,000
Dilutive effect of equity awards, based on the treasury stock method (in shares)	1,900,000			1,500,000			1,700,000	1,400,000
Weighted average shares assuming dilution (in shares)	75,800,000			75,500,000			75,600,000	75,300,000
Antidilutive options excluded from computation of diluted net income per share (in shares)	0.0			100,000			200,000	300,000
Stock repurchase program, shares authorized (in shares)									848,000
Stock repurchase program, shares purchased (in shares)	0						761,500
Stock purchase program, cost of shares purchased			$ 115.5			$ 83.1	$ 115.5
Stock repurchase program, average price per share (in dollars per share)							$ 151.65