Basis of Presentation and Significant Accounting Policies - Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	$ 433.1	$ 248.0
Property & Casualty
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	24.5	24.4
Life & Retirement
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	403.5	219.4
Supplemental & Group Benefits
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	$ 5.1	$ 4.2