Basis of Presentation - Reporting and Accounting Policies - Allowance for Credit Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Allowance for Credit Loss [Abstract]
Premium Receivable, Allowance for Credit Loss, Beginning Balance	$ 245.2	$ 437.9	$ 356.2	$ 283.2
Allowance acquired during period	0.0	0.0	3.5	0.0
Increase in allowance	107.2	(3.6)	255.5	362.9
Write-offs	(95.1)	(121.1)	(357.9)	(332.9)
Allowance for credit loss, end of period	$ 257.3	$ 313.2	$ 257.3	$ 313.2