Net Income Per Share and Stockholders' Equity, Calculation of EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator:
Net income	$ 25.4	$ 19.2	$ 28.0	$ 21.7	$ 20.7	$ 25.7	$ 72.6	$ 68.1
Denominator — Weighted-average common shares outstanding (in thousands):
Basic (shares)	15,956			16,573			16,042	16,654
Add: dilutive effect of non-vested common shares, restricted stock units, performance shares and stock options (shares)	169			210			193	228
Diluted (shares)	16,125			16,783			16,235	16,882
Net income per common share:
Basic (in dollars per share)	$ 1.59			$ 1.31			$ 4.52	$ 4.09
Earnings per common share, Diluted:
Diluted (in dollars per share)	$ 1.57			$ 1.29			$ 4.47	$ 4.03