Fair Value of Financial Instruments - Financial Instrument Not Carried at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 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	34.9	33.8
Deposit asset on reinsurance	0.0	0.0
Financial Liabilities
Policyholders' account balances	0.0	0.0
Reverse repurchase agreement	51.8
Other policyholder funds	933.7	908.7
Long-term debt	569.8	571.4
Level 3
Investments
Other investments	187.1	187.9
Deposit asset on reinsurance	2,192.2	2,259.6
Financial Liabilities
Policyholders' account balances	4,657.8	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	218.7	218.4
Deposit asset on reinsurance	2,472.4	2,496.6
Financial Liabilities
Policyholders' account balances	5,052.6	4,996.3
Reverse repurchase agreement	50.0
Other policyholder funds	941.1	916.0
Long-term debt	546.2	546.0
Fair Value
Investments
Other investments	222.0	221.7
Deposit asset on reinsurance	2,192.2	2,259.6
Financial Liabilities
Policyholders' account balances	4,657.8	4,861.8
Reverse repurchase agreement	51.8
Other policyholder funds	941.1	916.0
Long-term debt	$ 569.8	$ 571.4