Reserves - Roll Forward of Claims and Benefits Payable (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Claims and benefits payable, at beginning of period	$ 2,210.0	$ 1,523.0
Less: Reinsurance ceded and other	(1,228.8)	(744.1)
Net claims and benefits payable, at beginning of period	981.2	778.9
Incurred losses and loss adjustment expenses related to:
Current year	1,293.1	1,070.3
Prior years	(15.0)	19.7
Total incurred losses and loss adjustment expenses	1,278.1	1,090.0
Paid losses and loss adjustment expenses related to:
Current year	708.4	631.6
Prior years	438.6	383.5
Total paid losses and loss adjustment expenses	1,147.0	1,015.1
Net claims and benefits payable, at end of period	1,112.3	853.8
Plus: Reinsurance ceded and other	908.0	600.8
Claims and benefits payable, at end of period	2,020.3	1,454.6
Ceded to U.S. Government | Not Rated
Paid losses and loss adjustment expenses related to:
Reinsurance recoverables	$ 124.1	$ 56.0