Long-Duration Insurance Contracts - Reconciliation of Policyholder Account Balances to the Balance Sheet (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024	Dec. 31, 2023
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 5,079.6	$ 5,100.3
Indexed Universal Life
Policyholder Account Balance [Line Items]
Policyholders' account balances	78.2	72.9	$ 61.0	$ 57.8
Experience life
Policyholder Account Balance [Line Items]
Policyholders' account balances	57.4	58.0	60.1	61.2
Fixed Account Annuities
Policyholder Account Balance [Line Items]
Policyholders' account balances	4,495.9	4,508.4	4,519.9	4,556.0
Fixed Indexed Account Annuities
Policyholder Account Balance [Line Items]
Policyholders' account balances	399.7	409.5	437.4	449.0
SPIA (non-life contingent)
Policyholder Account Balance [Line Items]
Policyholders' account balances	28.3	28.6	$ 31.6	$ 32.6
Reconciling items
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 20.1	$ 22.9