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Interim Financial Information - Basis of Presentation (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
Business And Organization [Line Items]
Out-of-period adjustment	$ 10.6
Net income (loss)	$ 30.2	$ 49.2	$ 60.0	$ 87.9
Diluted earnings (loss) per common share (in dollars per share)	$ 1.63	$ 2.64	$ 3.25	$ 4.73
Restatement Adjustment
Business And Organization [Line Items]
Net income (loss)	$ (9.7)
Diluted earnings (loss) per common share (in dollars per share)	$ (0.53)

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