Net Income Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Earnings Per Share [Abstract]
Net income	$ 51.0	$ 37.6	$ 150.7	$ 104.5
Weighted average common shares outstanding (in shares)	74,200,000	73,300,000	73,800,000	73,000,000
Dilutive effect of stock options, stock appreciation rights and performance share awards, based on the treasury stock method (in shares)	1,700,000	1,700,000	2,000,000	1,700,000
Weighted average shares assuming dilution (in shares)	75,900,000	75,000,000	75,800,000	74,700,000
Antidilutive options excluded from computation of diluted net income per share (in shares)	500,000	100,000	400,000	100,000
Stock repurchase program, shares authorized					800,000
Stock repurchase program, shares purchased	0		325,000
Stock repurchase program, cost of shares purchased	$ 0.0		$ 26.9
Stock repurchase program, average price per share	$ 0.00		$ 82.84