Net Income Per Share and Stockholders' Equity, Calculation of EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net income	$ 19.9	$ 14.9	$ 60.6	$ 67.2
Denominator — Weighted-average common shares outstanding (in thousands):
Basic (shares)	16,834	17,841	17,072	17,858
Add: dilutive effect of non-vested common shares, restricted stock units, performance shares and stock options (shares)	326	334	291	323
Diluted (shares)	17,160	18,175	17,363	18,181
Net income per common share:
Basic (in dollars per share)	$ 1.18	$ 0.84	$ 3.55	$ 3.76
Earnings per common share, Diluted:
Diluted (in dollars per share)	$ 1.16	$ 0.82	$ 3.49	$ 3.70