Segment Information - Additional Significant Financial Information by Segment (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Assets	$ 15,266.6	$ 14,487.8	$ 14,049.9
Intersegment eliminations
Segment Reporting Information [Line Items]
Assets	(20.5)	(23.8)	(62.4)
Property & Casualty | Operating Segments
Segment Reporting Information [Line Items]
Assets	1,372.1	1,272.3	1,218.1
Life & Retirement | Operating Segments
Segment Reporting Information [Line Items]
Assets	12,204.5	11,670.7	11,365.0
Supplemental & Group Benefits | Operating Segments
Segment Reporting Information [Line Items]
Assets	1,401.7	1,377.6	1,338.8
Corporate & Other
Segment Reporting Information [Line Items]
Assets	$ 308.8	$ 191.0	$ 190.4