Long-Duration Insurance Contracts - Deferred Policy Acquisition Costs, Reconciliation to Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Deferred Policy Acquisition Cost [Line Items]
Deferred policy acquisition costs	$ 334.9		$ 330.6
Whole Life
Deferred Policy Acquisition Cost [Line Items]
Deferred policy acquisition costs	21.9	$ 21.6	20.9	$ 19.1	$ 17.8
Term Life
Deferred Policy Acquisition Cost [Line Items]
Deferred policy acquisition costs	32.0	31.4	30.0	27.5	25.6
Experience Life
Deferred Policy Acquisition Cost [Line Items]
Deferred policy acquisition costs	5.7	5.7	5.8	6.0	6.2
Limited Pay Whole Life
Deferred Policy Acquisition Cost [Line Items]
Deferred policy acquisition costs	7.2	7.0	6.7	5.6	4.4
Indexed Universal Life
Deferred Policy Acquisition Cost [Line Items]
Deferred policy acquisition costs	16.4	16.1	15.4	13.7	12.9
Supplemental health
Deferred Policy Acquisition Cost [Line Items]
Deferred policy acquisition costs	7.5	7.0	6.2	4.9	4.3
Total Annuities
Deferred Policy Acquisition Cost [Line Items]
Deferred policy acquisition costs	215.0	$ 217.2	221.1	$ 223.3	$ 223.1
Reconciling items
Deferred Policy Acquisition Cost [Line Items]
Deferred policy acquisition costs	$ 29.2		$ 24.5