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Earnings (Loss) Per Share (Details) - USD ($) $ / shares in Units, $ 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
Earnings Per Share [Abstract]
Weighted average common shares outstanding (in shares)									17,917,195	19,452,560	20,419,858
Dilutive effect of employee stock purchases, net of assumed repurchase of treasury stock (in shares)									18,879	8,492	5,041
Weighted average dilutive common shares outstanding (in shares)									17,936,074	19,461,052	20,424,899
Basic:
Net income (loss)	$ 48.1	$ 38.0	$ 49.2	$ 38.6	$ 30.7	$ 34.8	$ 56.5	$ 35.8	$ 174.0	$ 157.8	$ 213.4
Less: Earnings (loss) allocated to participating securities									6.4	5.4	7.5
Net income (loss) available to basic common shares									$ 167.6	$ 152.4	$ 205.9
Basic earnings (loss) per common share (in dollars per share)	$ 2.57	$ 2.04	$ 2.65	$ 2.09	$ 1.62	$ 1.74	$ 2.72	$ 1.70	$ 9.35	$ 7.83	$ 10.08
Diluted:
Net income (loss)	$ 48.1	$ 38.0	$ 49.2	$ 38.6	$ 30.7	$ 34.8	$ 56.5	$ 35.8	$ 174.0	$ 157.8	$ 213.4
Less: Earnings (loss) allocated to participating securities									6.4	5.4	7.5
Net income (loss) available to diluted common shares									$ 167.6	$ 152.4	$ 205.9
Diluted earnings (loss) per common share (in dollars per share)	$ 2.57	$ 2.04	$ 2.64	$ 2.08					$ 9.34	$ 7.83	$ 10.08

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