Net Income Per Common Share Computations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share [Abstract]
Net income	$ 108.1	$ 65.2	$ 46.4
Net income allocated to participating securities	(1.0)	(0.6)	(0.3)
Numerator for basic net income per common share	107.1	64.6	46.1
Numerator for diluted income per common share	$ 108.1	$ 65.2	$ 46.4
Denominator for net income per common share:
Basic (in shares)	16.4	17.2	17.3
Plus dilutive effect of stock options and restricted stock (in shares)	0.2	0.1	0.1
Plus dilutive effect of participating securities (in shares)	0.2	0.3	0.2
Diluted (in shares)	16.8	17.6	17.6
Income per common share:
Basic (in dollars per share)	$ 6.52	$ 3.75	$ 2.67
Diluted (in dollars per share)	$ 6.42	$ 3.71	$ 2.64