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, 2018	Mar. 31, 2017
Numerator:
Net income	$ 25.7	$ 36.0
Denominator — Weighted-average common shares outstanding (in thousands):
Basic (shares)	16,707	17,385
Add: dilutive effect of non-vested common shares, restricted stock units, performance shares and stock options (shares)	324	218
Diluted (shares)	17,031	17,603
Net income per common share:
Basic (in dollars per share)	$ 1.54	$ 2.07
Earnings per common share, Diluted:
Diluted (in dollars per share)	$ 1.51	$ 2.04