Reserves - Roll Forward of Claims and Benefits Payable (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Claims and benefits payable, at beginning of period	$ 1,989.2	$ 2,210.0
Less: Reinsurance ceded and other	(886.6)	(1,228.8)
Net claims and benefits payable, at beginning of period	1,102.6	981.2
Incurred losses and loss adjustment expenses related to:
Current year	1,384.2	1,293.1
Prior years	(65.0)	(15.0)
Total incurred losses and loss adjustment expenses	1,319.2	1,278.1
Paid losses and loss adjustment expenses related to:
Current year	731.5	708.4
Prior years	480.8	438.6
Total paid losses and loss adjustment expenses	1,212.3	1,147.0
Net claims and benefits payable, at end of period	1,209.5	1,112.3
Plus: Reinsurance ceded and other	905.7	908.0
Claims and benefits payable, at end of period	2,115.2	2,020.3
Ceded to U.S. Government | Not Rated
Liability for Claims and Claims Adjustment Expense [Line Items]
Reinsurance recoverables	$ 104.0	$ 124.1