Basis of Presentation and Significant Accounting Policies - Schedule of Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	$ 358.2	$ 347.2
Property & Casualty
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	40.0	34.4
Life & Retirement
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	303.7	301.4
Supplemental & Group Benefits
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	$ 14.5	$ 11.4