Long-Duration Insurance Contracts - Reconciliation of Policyholder Account Balances to the Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2025	Jun. 30, 2025	Dec. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 5,092.0		$ 5,100.3
Indexed Universal Life
Policyholder Account Balance [Line Items]
Policyholders' account balances	87.2	$ 82.3	72.9	$ 67.4	$ 65.2	$ 57.8
Experience life
Policyholder Account Balance [Line Items]
Policyholders' account balances	55.7	56.5	58.0	58.8	59.4	61.2
Fixed Account Annuities
Policyholder Account Balance [Line Items]
Policyholders' account balances	4,514.6	4,495.1	4,508.4	4,525.6	4,509.5	4,556.0
Fixed Indexed Account Annuities
Policyholder Account Balance [Line Items]
Policyholders' account balances	382.3	388.6	409.5	413.6	426.6	449.0
SPIA (non-life contingent)
Policyholder Account Balance [Line Items]
Policyholders' account balances	27.4	$ 27.9	28.6	$ 30.1	$ 30.4	$ 32.6
Reconciling items
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 24.8		$ 22.9