Segment Information - Additional Significant Financial Information by Segment (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Segment Reporting Information [Line Items]
Assets	$ 15,489.7	$ 14,487.8
Corporate & Other
Segment Reporting Information [Line Items]
Assets	519.8	191.0
Operating Segments | Property & Casualty
Segment Reporting Information [Line Items]
Assets	1,397.8	1,272.3
Operating Segments | Life & Retirement
Segment Reporting Information [Line Items]
Assets	12,212.7	11,670.7
Operating Segments | Supplemental & Group Benefits
Segment Reporting Information [Line Items]
Assets	1,413.3	1,377.6
Intersegment eliminations
Segment Reporting Information [Line Items]
Assets	$ (53.9)	$ (23.8)