Basis of Presentation and Significant Accounting Policies - Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	$ 336.3	$ 330.6	[1]
Property & Casualty
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	29.3	24.5
Life & Retirement
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	297.7	299.5
Supplemental & Group Benefits
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	$ 9.3	$ 6.6

[1] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements