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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net income	$ 21.7	$ 19.9	$ 68.1	$ 60.6
Denominator — Weighted-average common shares outstanding (in thousands):
Basic (shares)	16,573	16,834	16,654	17,072
Add: dilutive effect of non-vested common shares, restricted stock units, performance shares and stock options (shares)	210	326	228	291
Diluted (shares)	16,783	17,160	16,882	17,363
Net income per common share:
Basic (in dollars per share)	$ 1.31	$ 1.18	$ 4.09	$ 3.55
Earnings per common share, Diluted:
Diluted (in dollars per share)	$ 1.29	$ 1.16	$ 4.03	$ 3.49