Net Income Per Share and Stockholders' Equity, Calculation of EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator:
Net income	$ 28.0	$ 25.7
Denominator — Weighted-average common shares outstanding (in thousands):
Basic (shares)	16,108	16,707
Add: dilutive effect of non-vested common shares, restricted stock units, performance shares and stock options (shares)	264	324
Diluted (shares)	16,372	17,031
Net income per common share:
Basic (in dollars per share)	$ 1.74	$ 1.54
Earnings per common share, Diluted:
Diluted (in dollars per share)	$ 1.71	$ 1.51