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, 2017	Mar. 31, 2016
Numerator:
Net income	$ 36.0	$ 26.3
Denominator — Weighted-average common shares outstanding (in thousands):
Basic (shares)	17,385	17,864
Add: dilutive effect of non-vested common shares, restricted stock units, performance shares and stock options (shares)	218	336
Diluted (shares)	17,603	18,200
Net income per common share:
Basic (in dollars per share)	$ 2.07	$ 1.47
Earnings per common share, Diluted:
Diluted (in dollars per share)	$ 2.04	$ 1.44