Fair Value of Financial Instruments - Financial Instrument Not Carried at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
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	39.3	37.0
Deposit asset on reinsurance	0.0	0.0
Financial Liabilities
Policyholders' account balances	0.0	0.0
Reverse repurchase agreement		12.4
Other policyholder funds	1,043.2	992.9
Long-term debt	873.6	575.1
Level 3
Investments
Other investments	179.8	187.3
Deposit asset on reinsurance	2,185.5	2,121.9
Financial Liabilities
Policyholders' account balances	4,706.9	4,710.1
Reverse repurchase agreement		0.0
Other policyholder funds	3.0	2.8
Long-term debt	0.0	0.0
Carrying Amount
Investments
Other investments	215.9	221.0
Deposit asset on reinsurance	2,402.3	2,434.3
Financial Liabilities
Policyholders' account balances	5,016.3	5,020.2
Reverse repurchase agreement		12.0
Other policyholder funds	1,046.2	995.7
Long-term debt	842.9	547.0
Fair Value
Investments
Other investments	219.1	224.3
Deposit asset on reinsurance	2,185.5	2,121.9
Financial Liabilities
Policyholders' account balances	4,706.9	4,710.1
Reverse repurchase agreement		12.4
Other policyholder funds	1,046.2	995.7
Long-term debt	$ 873.6	$ 575.1