Basis of Presentation - Reporting and Accounting Policies - Allowance for Credit Loss (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Allowance for Credit Loss [Abstract]
Premium Receivable, Allowance for Credit Loss, Beginning Balance	$ 265.3	$ 344.7	$ 356.2	$ 283.2
Allowance acquired during period	3.5	0.0	3.5	0.0
Increase in allowance	87.4	196.2	148.3	366.5
Write-offs	(111.0)	(103.0)	(262.8)	(211.8)
Allowance for credit loss, end of period	$ 245.2	$ 437.9	$ 245.2	$ 437.9