Long-Duration Insurance Contracts - Reconciliation of Policyholder Account Balances to the Balance Sheet (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022	Dec. 31, 2021
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 5,234.6	$ 5,260.6
Indexed Universal Life
Policyholder Account Balance [Line Items]
Policyholders' account balances	49.9	47.6	$ 41.0	$ 39.1
Experience Life
Policyholder Account Balance [Line Items]
Policyholders' account balances	63.5	64.3	65.9	66.2
Fixed Account Annuities
Policyholder Account Balance [Line Items]
Policyholders' account balances	4,577.4	4,591.1	4,548.2	4,532.7
Fixed Indexed Account Annuities
Policyholder Account Balance [Line Items]
Policyholders' account balances	498.4	510.3	518.2	522.6
SPIA (non-life contingent)
Policyholder Account Balance [Line Items]
Policyholders' account balances	33.8	34.4	$ 37.0	$ 37.7
Reconciling items
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 11.6	$ 12.9