Long-Duration Insurance Contracts - Reconciliation of Policyholder Account Balances to the Balance Sheet (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 5,137.6	$ 5,187.0
Indexed Universal Life Insurance
Policyholder Account Balance [Line Items]
Policyholders' account balances	61.0	57.8	$ 49.9	$ 47.6
Experience Life Insurance
Policyholder Account Balance [Line Items]
Policyholders' account balances	60.1	61.2	63.5	64.3
Fixed Annuity
Policyholder Account Balance [Line Items]
Policyholders' account balances	4,519.9	4,556.0	4,577.4	4,591.1
Indexed Annuity
Policyholder Account Balance [Line Items]
Policyholders' account balances	437.4	449.0	498.4	510.3
Non-Life Contingent Single-Premium Immediate Annuity Insurance
Policyholder Account Balance [Line Items]
Policyholders' account balances	31.6	32.6	$ 33.8	$ 34.4
Reconciling items
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 27.6	$ 30.4