Long-Duration Insurance Contracts - Deferred Policy Acquisition Costs, Reconciliation to Balance Sheet (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Deferred Policy Acquisition Cost [Line Items]
Deferred policy acquisition costs	$ 337.3	$ 336.3
Whole Life Insurance
Deferred Policy Acquisition Cost [Line Items]
Deferred policy acquisition costs	22.6	22.3	$ 21.2	$ 20.9
Term Life Insurance
Deferred Policy Acquisition Cost [Line Items]
Deferred policy acquisition costs	32.9	32.6	30.7	30.0
Experience Life Insurance
Deferred Policy Acquisition Cost [Line Items]
Deferred policy acquisition costs	5.7	5.7	5.8	5.8
Limited Pay Whole Life Insurance
Deferred Policy Acquisition Cost [Line Items]
Deferred policy acquisition costs	7.5	7.4	6.8	6.7
Indexed Universal Life Insurance
Deferred Policy Acquisition Cost [Line Items]
Deferred policy acquisition costs	17.1	16.8	15.7	15.4
Supplemental health
Deferred Policy Acquisition Cost [Line Items]
Deferred policy acquisition costs	8.7	8.2	6.7	6.2
Total Annuities
Deferred Policy Acquisition Cost [Line Items]
Deferred policy acquisition costs	212.9	214.0	$ 219.7	$ 221.1
Reconciling items
Deferred Policy Acquisition Cost [Line Items]
Deferred policy acquisition costs	$ 29.9	$ 29.3