Basis of Presentation and Significant Accounting Policies - Schedule of Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	$ 347.2	$ 336.3
Property & Casualty
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	34.4	29.3
Life & Retirement
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	301.4	297.7
Supplemental & Group Benefits
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	$ 11.4	$ 9.3