Fair Value of Financial Instruments - Financial Instrument Not Carried at Fair Value (Details) - USD ($) $ in Millions	Sep. 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	36.2	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	963.3	908.7
Long-term debt	578.7	571.4
Level 3
Investments
Other investments	188.2	187.9
Deposit asset on reinsurance	2,268.1	2,259.6
Financial Liabilities
Policyholders' account balances	4,751.8	4,861.8
Reverse repurchase agreement	0.0
Other policyholder funds	2.8	7.3
Long-term debt	0.0	0.0
Carrying Amount
Investments
Other investments	221.1	218.4
Deposit asset on reinsurance	2,461.1	2,496.6
Financial Liabilities
Policyholders' account balances	5,042.0	4,996.3
Reverse repurchase agreement	45.0
Other policyholder funds	966.1	916.0
Long-term debt	546.7	546.0
Fair Value
Investments
Other investments	224.4	221.7
Deposit asset on reinsurance	2,268.1	2,259.6
Financial Liabilities
Policyholders' account balances	4,751.8	4,861.8
Reverse repurchase agreement	46.9
Other policyholder funds	966.1	916.0
Long-term debt	$ 578.7	$ 571.4