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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Numerator:
Net income	$ 4.7	$ 26.0	$ 40.7	$ 52.3
Denominator — Weighted-average common shares outstanding (in thousands):
Basic (shares)	17,003	17,871	17,193	17,867
Add: dilutive effect of non-vested common shares, restricted stock units, performance shares and stock options (shares)	198	323	225	327
Diluted (shares)	17,201	18,194	17,418	18,194
Net income per common share:
Basic (in dollars per share)	$ 0.28	$ 1.45	$ 2.37	$ 2.92
Earnings per common share, Diluted:
Diluted (in dollars per share)	$ 0.27	$ 1.43	$ 2.34	$ 2.87