Net Income Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Earnings Per Share [Abstract]
Net income	$ 55.4	$ 43.6
Weighted average common shares outstanding (in shares)	74,100,000	73,900,000
Dilutive effect of equity awards, based on the treasury stock method (in shares)	1,200,000	1,600,000
Weighted average shares assuming dilution (in shares)	75,300,000	75,500,000
Antidilutive options excluded from computation of diluted net income per share (in shares)	600,000	700,000
Stock repurchase program, shares authorized			800,000
Stock repurchase program, shares purchased	800,000
Stock purchase program, cost of shares purchased	$ 83.1	$ 47.9
Stock repurchase program, average price per share	$ 103.89