Net Income Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Earnings Per Share [Abstract]
Net income	$ 56.1	$ 38.8	$ 99.7	$ 99.7
Weighted average common shares outstanding (in shares)	73,600,000	73,900,000	73,800,000	73,600,000
Dilutive effect of equity awards, based on the treasury stock method (in shares)	1,400,000	1,900,000	1,400,000	2,100,000
Weighted average shares assuming dilution (in shares)	75,000,000	75,800,000	75,200,000	75,700,000
Antidilutive options excluded from computation of diluted net income per share (in shares)	900,000	500,000	800,000	300,000
Stock repurchase program, shares authorized					800,000
Stock repurchase program, shares purchased	260,000		800,000
Stock purchase program, cost of shares purchased	$ 22.9		$ 70.8
Stock repurchase program, average price per share	$ 88.03		$ 88.51