Business Segments - Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2025 USD ($) segments	Dec. 31, 2024 USD ($)	Mar. 31, 2024 USD ($)
Segment Reporting [Abstract]
Number of operating segments | segments	2
Segment Reporting, Asset Reconciling Item [Line Items]
Total Assets	$ 12,467.3	$ 12,630.4	$ 12,630.4
Operating Segments
Segment Reporting, Asset Reconciling Item [Line Items]
Total Assets	11,377.9		11,084.6
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting, Asset Reconciling Item [Line Items]
Total Assets	6,527.3		6,352.9
Operating Segments | Specialty Property & Casualty Insurance | Variable Interest Entity, Primary Beneficiary
Segment Reporting, Asset Reconciling Item [Line Items]
Total Assets	61.0		41.5
Operating Segments | Life Insurance
Segment Reporting, Asset Reconciling Item [Line Items]
Total Assets	4,850.6		4,731.7
Corporate and Other
Segment Reporting, Asset Reconciling Item [Line Items]
Total Assets	374.1		774.7
Non-Core Operations
Segment Reporting, Asset Reconciling Item [Line Items]
Total Assets	$ 715.3		$ 771.1