Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Basic and Diluted Earnings per share
Weighted average basic common shares outstanding (in shares)	17,776,888	17,961,555	17,770,619	17,889,572
Dilutive effect of employee stock purchases, net of assumed repurchase of treasury stock (in shares)	58,661	17,145	47,919	17,978
Weighted average dilutive common shares (in shares)	17,835,549	17,978,700	17,818,538	17,900,000	17,907,550
Basic:
Net income (loss)	$ 126.4	$ 38.0	$ 186.4	$ 125.9
Less: Earnings (loss) allocated to participating securities	4.6	1.4	6.7	4.7
Net income (loss) available to basic common shares	$ 121.9	$ 36.7	$ 179.7	$ 121.2
Basic earnings (loss) per common share (in dollars per share)	$ 6.86	$ 2.04	$ 10.11	$ 6.78
Diluted:
Net income (loss)	$ 126.4	$ 38.0	$ 186.4	$ 125.9
Less: Earnings (loss) allocated to participating securities	4.5	1.4	6.7	4.7
Net income (loss) available to diluted common shares	$ 121.9	$ 36.7	$ 179.7	$ 121.2
Diluted earnings (loss) per common share (in dollars per share)	$ 6.83	$ 2.04	$ 10.08	$ 6.77