Fair Value of Financial Instruments - Financial Instrument Not Carried at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Financial Assets
Policy loans	$ 136.3	$ 138.1
Other investments	116.7	99.0
Level 1
Financial Assets
Policy loans	0.0	0.0
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
Policy loans	0.0	0.0
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	1,073.1	1,043.2
Long-term debt	620.7	629.1
Level 3
Financial Assets
Policy loans	139.3	141.2
Other investments	54.1	38.2
Deposit asset on reinsurance	2,087.9	2,143.1
Financial Liabilities
Policyholders' account balances	4,625.4	4,791.1
Other policyholder funds	2.9	3.0
Long-term debt	0.0	0.0
Carrying Amount
Financial Assets
Policy loans	136.3	138.1
Other investments	92.3	77.4
Deposit asset on reinsurance	2,345.4	2,369.6
Financial Liabilities
Policyholders' account balances	4,962.9	4,984.9
Other policyholder funds	1,076.0	1,046.2
Long-term debt	594.2	593.4
Fair Value
Financial Assets
Policy loans	139.3	141.2
Other investments	92.3	77.5
Deposit asset on reinsurance	2,087.9	2,143.1
Financial Liabilities
Policyholders' account balances	4,625.4	4,791.1
Other policyholder funds	1,076.0	1,046.2
Long-term debt	$ 620.7	$ 629.1