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, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net income	$ 19.2	$ 28.0	$ 20.7	$ 25.7	$ 47.2	$ 46.4
Denominator — Weighted-average common shares outstanding (in thousands):
Basic (shares)	16,063		16,685		16,085	16,696
Add: dilutive effect of non-vested common shares, restricted stock units, performance shares and stock options (shares)	177		220		213	266
Diluted (shares)	16,240		16,905		16,298	16,962
Net income per common share:
Basic (in dollars per share)	$ 1.19		$ 1.24		$ 2.93	$ 2.78
Earnings per common share, Diluted:
Diluted (in dollars per share)	$ 1.18		$ 1.22		$ 2.89	$ 2.74