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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net income	$ (10.6)	$ 25.4	$ 19.2	$ 28.0	$ 23.6	$ 21.7	$ 20.7	$ 25.7	$ 62.0	$ 91.7	$ 45.4
Denominator – Weighted-average common shares outstanding (in thousands):
Basic (in shares)									15,997	16,585	16,996
Add: dilutive effect of non-vested common shares, restricted stock units and performance shares1									206	289	263
Diluted (in shares)									16,203	16,874	17,259
Basic (in dollars per share)	$ (0.66)	$ 1.59	$ 1.19	$ 1.74	$ 1.44	$ 1.31	$ 1.24	$ 1.54	$ 3.88	$ 5.53	$ 2.67
Net (loss) income per common share, Diluted (in dollars per share	$ (0.66)	$ 1.57	$ 1.18	$ 1.71	$ 1.41	$ 1.29	$ 1.22	$ 1.51	$ 3.83	$ 5.43	$ 2.63
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount									0		52