Fair Value of Financial Instruments - Financial Instrument Not Carried at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Level 1
Financial Assets
Other investments	$ 0.0	$ 0.0
Deposit asset on reinsurance	0.0	0.0
Financial Liabilities
Policyholders' account balances	0.0	0.0
Other policyholder funds	0.0	0.0
Long-term debt	0.0	0.0
Level 2
Financial Assets
Other investments	38.2	39.3
Deposit asset on reinsurance	0.0	0.0
Financial Liabilities
Policyholders' account balances	0.0	0.0
Other policyholder funds	1,058.1	1,043.2
Long-term debt	619.8	629.1
Level 3
Financial Assets
Other investments	192.7	179.4
Deposit asset on reinsurance	2,094.9	2,143.1
Financial Liabilities
Policyholders' account balances	4,640.3	4,791.1
Other policyholder funds	3.0	3.0
Long-term debt	0.0	0.0
Carrying Amount
Financial Assets
Other investments	227.8	215.5
Deposit asset on reinsurance	2,361.8	2,369.6
Financial Liabilities
Policyholders' account balances	4,978.5	4,984.9
Other policyholder funds	1,061.1	1,046.2
Long-term debt	593.8	593.4
Fair Value
Financial Assets
Other investments	230.9	218.7
Deposit asset on reinsurance	2,094.9	2,143.1
Financial Liabilities
Policyholders' account balances	4,640.3	4,791.1
Other policyholder funds	1,061.1	1,046.2
Long-term debt	$ 619.8	$ 629.1