Net Income Per Share and Stockholders' Equity, Calculation of EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator:
Net income	$ 20.7	$ 4.7	$ 46.4	$ 40.7
Denominator — Weighted-average common shares outstanding (in thousands):
Basic (shares)	16,685	17,003	16,696	17,193
Add: dilutive effect of non-vested common shares, restricted stock units, performance shares and stock options (shares)	220	198	266	225
Diluted (shares)	16,905	17,201	16,962	17,418
Net income per common share:
Basic (in dollars per share)	$ 1.24	$ 0.28	$ 2.78	$ 2.37
Earnings per common share, Diluted:
Diluted (in dollars per share)	$ 1.22	$ 0.27	$ 2.74	$ 2.34