Business Segments - Segment Assets (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Total Assets	$ 12,742.7	$ 13,313.6	[1]
Operating Segments
Segment Reporting Information [Line Items]
Total Assets	11,044.0	11,543.3
Corporate, Non-Segment
Segment Reporting Information [Line Items]
Total Assets	623.7	545.4
Segment Reconciling Items
Segment Reporting Information [Line Items]
Total Assets	1,075.0	1,224.9
Specialty Property & Casualty Insurance | Operating Segments
Segment Reporting Information [Line Items]
Total Assets	6,145.9	6,535.3
Life Insurance | Operating Segments
Segment Reporting Information [Line Items]
Total Assets	$ 4,898.1	$ 5,008.0

[1] As of January 1, 2023, the Company adopted ASU 2018-12 using the modified retrospective method applied as of the transition date of January 1, 2021. Prior period amounts in the financial statements have been recast to reflect application of the new guidance. See Note 2 to the Consolidated Financial Statements for additional information.