Net Income Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Earnings Per Share [Abstract]
Net income	$ 56.3	$ 66.1	$ 55.4	$ 55.2	$ 56.1	$ 43.6	$ 177.8	$ 154.9
Weighted average common shares outstanding (in shares)	74,000,000.0			73,900,000			73,900,000	73,900,000
Dilutive effect of equity awards, based on the treasury stock method (in shares)	1,500,000			1,800,000			1,400,000	1,500,000
Weighted average shares assuming dilution (in shares)	75,500,000			75,700,000			75,300,000	75,400,000
Antidilutive options excluded from computation of diluted net income per share (in shares)	100,000			100,000			300,000	400,000
Stock repurchase program, shares authorized									800,000
Stock repurchase program, shares purchased							800,000
Stock purchase program, cost of shares purchased			$ 83.1		$ 22.9	$ 47.9	$ 83.1
Stock repurchase program, average price per share							$ 103.89