Long-Duration Insurance Contracts - Reconciliation of Policyholder Account Balances to the Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 5,128.6		$ 5,187.0
Indexed Universal Life Insurance
Policyholder Account Balance [Line Items]
Policyholders' account balances	67.4	$ 65.2	57.8	$ 54.7	$ 52.0	$ 47.6
Experience Life Insurance
Policyholder Account Balance [Line Items]
Policyholders' account balances	58.8	59.4	61.2	62.1	63.0	64.3
Fixed Annuity
Policyholder Account Balance [Line Items]
Policyholders' account balances	4,525.6	4,509.5	4,556.0	4,568.5	4,563.5	4,591.1
Indexed Annuity
Policyholder Account Balance [Line Items]
Policyholders' account balances	413.6	426.6	449.0	468.0	481.2	510.3
Non-Life Contingent Single-Premium Immediate Annuity Insurance
Policyholder Account Balance [Line Items]
Policyholders' account balances	30.1	$ 30.4	32.6	$ 33.1	$ 33.5	$ 34.4
Reconciling items
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 33.1		$ 30.4