Net Income Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Mar. 31, 2019
Earnings Per Share [Abstract]
Net income, as reported, for basic net income per share	$ 206.9	$ 150.7	$ 143.6
Weighted average common shares outstanding (in shares)	73,900,000	73,900,000	73,300,000
Assumed stock options exercised and awards vested, based on the treasury stock method (in shares)	1,500,000	1,900,000	1,700,000
Weighted average shares assuming dilution (in shares)	75,400,000	75,800,000	75,000,000
Antidilutive options excluded from computation of diluted net income per share (in shares)	400,000	400,000	100,000
Share repurchase program, number of shares authorized		800,000		800,000
Shares purchased under share repurchase program (in shares)	800,000
Shares purchased under share repurchase program	$ 70.8	$ 74.4	$ 52.2
Shares purchased under share repurchase program, average cost per share	$ 88.51