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Business Segments - Narrative (Details) $ in Millions	9 Months Ended
	Sep. 30, 2025 USD ($) segments	Dec. 31, 2024 USD ($)	Sep. 30, 2024 USD ($)
Segment Reporting [Abstract]
Number of operating segments | segments	2
Segment Reporting, Asset Reconciling Item [Line Items]
Total Assets	$ 12,443.4	$ 12,630.4	$ 12,630.4
Operating Segments
Segment Reporting, Asset Reconciling Item [Line Items]
Total Assets	11,521.1		11,084.6
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting, Asset Reconciling Item [Line Items]
Total Assets	6,642.1		6,352.9
Operating Segments | Specialty Property & Casualty Insurance | Variable Interest Entity, Primary Beneficiary
Segment Reporting, Asset Reconciling Item [Line Items]
Total Assets	70.0		41.5
Operating Segments | Life Insurance
Segment Reporting, Asset Reconciling Item [Line Items]
Total Assets	4,879.0		4,731.7
Corporate and Other
Segment Reporting, Asset Reconciling Item [Line Items]
Total Assets	262.9		774.7
Non-Core Operations
Segment Reporting, Asset Reconciling Item [Line Items]
Total Assets	$ 659.4		$ 771.1

X

- Definition

Amount of asset recognized for present right to economic benefit.

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