Net Income Per Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Earnings Per Share [Abstract]
Net income	$ 1.5	$ 31.0	$ 62.2	$ 95.7
Weighted average common shares outstanding (in shares)	72.2	71.0	71.9	70.8
Dilutive effect of stock options, stock appreciation rights and performance share awards, based on the treasury stock method (in shares)	1.7	1.6	1.7	1.7
Weighted average shares assuming dilution (in shares)	73.9	72.6	73.6	72.5
Antidilutive options excluded from computation of diluted net income per share (in shares)	0.8	0.6	0.6	0.5