Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Basic and Diluted Earnings per share
Weighted average basic common shares outstanding (in shares)	17,771,447	17,908,039	17,767,449	17,852,985
Dilutive effect of employee stock purchases, net of assumed repurchase of treasury stock (in shares)	40,166	22,241	42,488	24,691
Weighted average dilutive common shares (in shares)	17,811,613	17,930,280	17,809,937	17,877,676
Basic:
Net income (loss)	$ 30.2	$ 49.2	$ 60.0	$ 87.9
Less: Earnings (loss) allocated to participating securities	1.1	1.8	2.2	3.3
Net income (loss) available to basic common shares	$ 29.1	$ 47.4	$ 57.8	$ 84.6
Basic earnings (loss) per common share (in dollars per share)	$ 1.64	$ 2.65	$ 3.25	$ 4.74
Diluted:
Net income (loss)	$ 30.2	$ 49.2	$ 60.0	$ 87.9
Less: Earnings (loss) allocated to participating securities	1.1	1.8	2.1	3.3
Net income (loss) available to diluted common shares	$ 29.1	$ 47.4	$ 57.8	$ 84.6
Diluted earnings (loss) per common share (in dollars per share)	$ 1.63	$ 2.64	$ 3.25	$ 4.73