Fair Value of Financial Instruments - Financial Instrument Not Carried at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Level 1
Investments
Other investments	$ 0.0	$ 0.0
Deposit asset on reinsurance	0.0	0.0
Financial Liabilities
Policyholders' account balances	0.0	0.0
Reverse repurchase agreement	0.0
Other policyholder funds	0.0	0.0
Long-term debt	0.0	0.0
Level 2
Investments
Other investments	35.3	33.8
Deposit asset on reinsurance	0.0	0.0
Financial Liabilities
Policyholders' account balances	0.0	0.0
Reverse repurchase agreement	46.9
Other policyholder funds	943.7	908.7
Long-term debt	569.6	571.4
Level 3
Investments
Other investments	187.2	187.9
Deposit asset on reinsurance	2,148.0	2,259.6
Financial Liabilities
Policyholders' account balances	4,747.1	4,861.8
Reverse repurchase agreement	0.0
Other policyholder funds	7.4	7.3
Long-term debt	0.0	0.0
Carrying Amount
Investments
Other investments	219.1	218.4
Deposit asset on reinsurance	2,466.2	2,496.6
Financial Liabilities
Policyholders' account balances	5,037.3	4,996.3
Reverse repurchase agreement	45.0
Other policyholder funds	951.1	916.0
Long-term debt	546.5	546.0
Fair Value
Investments
Other investments	222.5	221.7
Deposit asset on reinsurance	2,148.0	2,259.6
Financial Liabilities
Policyholders' account balances	4,747.1	4,861.8
Reverse repurchase agreement	46.9
Other policyholder funds	951.1	916.0
Long-term debt	$ 569.6	$ 571.4