Segment Information - Additional Significant Financial Information by Segment (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Assets	$ 14,487.8	$ 14,049.9	$ 13,306.1
Intersegment eliminations
Segment Reporting Information [Line Items]
Assets	(23.8)	(62.4)	(64.8)
Property & Casualty | Operating Segments
Segment Reporting Information [Line Items]
Assets	1,272.3	1,218.1	1,083.8
Life & Retirement | Operating Segments
Segment Reporting Information [Line Items]
Assets	11,670.7	11,365.0	10,754.4
Supplemental & Group Benefits | Operating Segments
Segment Reporting Information [Line Items]
Assets	1,377.6	1,338.8	1,359.3
Corporate & Other
Segment Reporting Information [Line Items]
Assets	$ 191.0	$ 190.4	$ 173.4