Reserves - Roll Forward of Claims and Benefits Payable (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Sep. 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	1,953.4	1,728.0
Prior years	(30.7)	32.5
Total incurred losses and loss adjustment expenses	1,922.7	1,760.5
Paid losses and loss adjustment expenses related to:
Current year	1,250.8	1,106.2
Prior years	511.8	449.8
Total paid losses and loss adjustment expenses	1,762.6	1,556.0
Net claims and benefits payable, at end of period	1,141.3	983.4
Plus: Reinsurance ceded and other	869.5	1,958.0
Claims and benefits payable, at end of period	2,010.8	2,941.4
Ceded to U.S. Government | Not Rated
Paid losses and loss adjustment expenses related to:
Reinsurance recoverables	$ 104.9	$ 960.4