Long-Duration Insurance Contracts - Reconciliation of Policyholder Account Balances to the Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 5,187.0	$ 5,260.6	[1]
Indexed Universal Life
Policyholder Account Balance [Line Items]
Policyholders' account balances	57.8	47.6		$ 39.1
Experience life
Policyholder Account Balance [Line Items]
Policyholders' account balances	61.2	64.3		66.2
Fixed Account Annuities
Policyholder Account Balance [Line Items]
Policyholders' account balances	4,556.0	4,591.1		4,532.7
Fixed Indexed Account Annuities
Policyholder Account Balance [Line Items]
Policyholders' account balances	449.0	510.3		522.6
SPIA (non-life contingent)
Policyholder Account Balance [Line Items]
Policyholders' account balances	32.6	34.4		$ 37.7
Reconciling items
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 30.4	$ 12.9

[1] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements