Interim Financial Information - Basis of Presentation (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Business And Organization [Line Items]
Out-of-period adjustment		$ 10.6
Net income (loss)	$ 126.4		$ 38.0	$ 186.4	$ 125.9
Diluted earnings (loss) per common share (in dollars per share)	$ 6.83		$ 2.04	$ 10.08	$ 6.77
Restatement Adjustment
Business And Organization [Line Items]
Net income (loss)		$ (9.7)
Diluted earnings (loss) per common share (in dollars per share)		$ (0.53)