Segment Information - Additional Significant Financial Information by Segment (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Segment Reporting Information
Assets	$ 14,396.1	$ 14,487.8
Corporate & Other
Segment Reporting Information
Assets	198.6	191.0
Operating Segments | Property & Casualty
Segment Reporting Information
Assets	1,302.5	1,272.3
Operating Segments | Life & Retirement
Segment Reporting Information
Assets	11,585.1	11,670.7
Operating Segments | Supplemental & Group Benefits
Segment Reporting Information
Assets	1,362.3	1,377.6
Intersegment eliminations
Segment Reporting Information
Assets	$ (52.4)	$ (23.8)