Business Segments - Narrative (Details) $ in Millions	6 Months Ended
	Jun. 30, 2025 USD ($) segments	Dec. 31, 2024 USD ($)	Jun. 30, 2024 USD ($)
Segment Reporting [Abstract]
Number of operating segments | segments	2
Segment Reporting, Asset Reconciling Item [Line Items]
Total Assets	$ 12,601.1	$ 12,630.4	$ 12,630.4
Operating Segments
Segment Reporting, Asset Reconciling Item [Line Items]
Total Assets	11,476.7		11,084.6
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting, Asset Reconciling Item [Line Items]
Total Assets	6,661.9		6,352.9
Operating Segments | Specialty Property & Casualty Insurance | Variable Interest Entity, Primary Beneficiary
Segment Reporting, Asset Reconciling Item [Line Items]
Total Assets	66.0		41.5
Operating Segments | Life Insurance
Segment Reporting, Asset Reconciling Item [Line Items]
Total Assets	4,814.8		4,731.7
Corporate and Other
Segment Reporting, Asset Reconciling Item [Line Items]
Total Assets	451.9		774.7
Non-Core Operations
Segment Reporting, Asset Reconciling Item [Line Items]
Total Assets	$ 672.5		$ 771.1