Fair Value of Financial Instruments - Financial Instrument Not Carried at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 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	37.5	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	1,002.9	992.9
Long-term debt	577.8	575.1
Level 3
Investments
Other investments	185.9	187.3
Deposit asset on reinsurance	2,141.4	2,121.9
Financial Liabilities
Policyholders' account balances	4,730.2	4,710.1
Reverse repurchase agreement		0.0
Other policyholder funds	2.9	2.8
Long-term debt	0.0	0.0
Carrying Amount
Investments
Other investments	220.2	221.0
Deposit asset on reinsurance	2,421.0	2,434.3
Financial Liabilities
Policyholders' account balances	5,096.5	5,020.2
Reverse repurchase agreement		12.0
Other policyholder funds	1,005.8	995.7
Long-term debt	547.2	547.0
Fair Value
Investments
Other investments	223.4	224.3
Deposit asset on reinsurance	2,141.4	2,121.9
Financial Liabilities
Policyholders' account balances	4,730.2	4,710.1
Reverse repurchase agreement		12.4
Other policyholder funds	1,005.8	995.7
Long-term debt	$ 577.8	$ 575.1