Net Income (Loss) Per Share and Stockholders' Equity, Calculation of EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net income (loss)	$ (15.2)	$ 19.9	$ 4.7	$ 36.0	$ 24.5	$ 14.9	$ 26.0	$ 26.3	$ 45.4	$ 91.7	$ (236.6)
Denominator – Weighted-average common shares outstanding (in thousands):
Basic (in shares)									16,996	17,813	17,201
Add: dilutive effect of non-vested common shares, restricted stock units and performance shares									263	220	0
Diluted (in shares)									17,259	18,033	17,201
Net (loss) income per common share, Basic (in dollars per share)	$ (0.90)	$ 1.18	$ 0.28	$ 2.07	$ 1.39	$ 0.84	$ 1.45	$ 1.47	$ 2.67	$ 5.15	$ (13.76)
Net (loss) income per common share, Diluted (in dollars per share	$ (0.90)	$ 1.16	$ 0.27	$ 2.04	$ 1.37	$ 0.82	$ 1.43	$ 1.44	$ 2.63	$ 5.09	$ (13.76)