Net Income 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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net income	$ 23.6	$ 21.7	$ 20.7	$ 25.7	$ (15.2)	$ 19.9	$ 4.7	$ 36.0	$ 91.7	$ 45.4	$ 91.7
Denominator – Weighted-average common shares outstanding (in thousands):
Basic (in shares)									16,585	16,996	17,813
Add: dilutive effect of non-vested common shares, restricted stock units and performance shares1									289	263	220
Diluted (in shares)									16,874	17,259	18,033
Basic (in dollars per share)	$ 1.44	$ 1.31	$ 1.24	$ 1.54	$ (0.90)	$ 1.18	$ 0.28	$ 2.07	$ 5.53	$ 2.67	$ 5.15
Net (loss) income per common share, Diluted (in dollars per share	$ 1.41	$ 1.29	$ 1.22	$ 1.51	$ (0.90)	$ 1.16	$ 0.27	$ 2.04	$ 5.43	$ 2.63	$ 5.09
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount										52	50