Long-Duration Insurance Contracts - Reconciliation of Policyholder Account Balances to the Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 5,204.0		$ 5,260.6
Indexed Universal Life
Policyholder Account Balance [Line Items]
Policyholders' account balances	54.7	$ 52.0	47.6	$ 44.8	$ 42.6	$ 39.1
Experience Life
Policyholder Account Balance [Line Items]
Policyholders' account balances	62.1	63.0	64.3	64.7	65.4	66.2
Fixed Account Annuities
Policyholder Account Balance [Line Items]
Policyholders' account balances	4,568.5	4,563.5	4,591.1	4,604.1	4,573.7	4,532.7
Fixed Indexed Account Annuities
Policyholder Account Balance [Line Items]
Policyholders' account balances	468.0	481.2	510.3	516.9	518.2	522.6
SPIA (non-life contingent)
Policyholder Account Balance [Line Items]
Policyholders' account balances	33.1	$ 33.5	34.4	$ 35.4	$ 36.3	$ 37.7
Reconciling items
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 17.6		$ 12.9