Reinsurance (Reinsurance Recoverable) (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Ceded Credit Risk [Line Items]
Total reinsurance recoverable	$ 6,649.2	$ 6,999.4
Ceded future policyholder benefits and expense
Ceded Credit Risk [Line Items]
Total reinsurance recoverable	339.9	354.3
Ceded unearned premium
Ceded Credit Risk [Line Items]
Total reinsurance recoverable	5,265.2	5,162.2
Ceded claims and benefits payable
Ceded Credit Risk [Line Items]
Total reinsurance recoverable	971.4	1,313.7
Ceded paid losses
Ceded Credit Risk [Line Items]
Total reinsurance recoverable	$ 72.7	$ 169.2