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Earnings (Loss) Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Weighted average common shares outstanding (in shares)	17,754,666	17,917,195	19,452,560
Dilutive effect of employee stock purchases, net of assumed repurchase of treasury stock (in shares)	51,912	18,879	8,492
Weighted average dilutive common shares outstanding (in shares)	17,806,578	17,936,074	19,461,052
Basic:
Net income (loss)	$ 286.5	$ 174.0	$ 157.8
Less: Earnings (loss) allocated to participating securities	10.3	6.4	5.4
Net income (loss) available to basic common shares	$ 276.2	$ 167.6	$ 152.4
Basic earnings (loss) per common share (in dollars per share)	$ 15.55	$ 9.35	$ 7.83
Diluted:
Net income (loss)	$ 286.5	$ 174.0	$ 157.8
Less: Earnings (loss) allocated to participating securities	10.3	6.4	5.4
Net income (loss) available to diluted common shares	$ 276.2	$ 167.6	$ 152.4
Diluted earnings (loss) per common share (in dollars per share)	$ 15.51	$ 9.34	$ 7.83

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