Business Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]
Earned premiums	$ 3,187,389	$ 3,098,185	$ 6,302,561	$ 6,110,566
Insurance
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]
Earned premiums		2,728,784		5,371,291
Insurance \| Operating Segments
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]
Earned premiums	2,826,030	2,728,784	5,591,522	5,371,291
Insurance \| Other liability
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]
Earned premiums	1,127,144	1,104,332	2,224,985	2,177,060
Insurance \| Short-tail lines
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]
Earned premiums	663,471	619,988	1,315,994	1,216,096
Insurance \| Auto
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]
Earned premiums	424,879	405,632	838,371	795,581
Insurance \| Workers' compensation
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]
Earned premiums	315,434	318,881	629,176	629,910
Insurance \| Professional liability
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]
Earned premiums	295,102	279,951	582,996	552,644
Reinsurance & Monoline Excess
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]
Earned premiums		369,401		739,275
Reinsurance & Monoline Excess \| Operating Segments
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]
Earned premiums	361,359	369,401	711,039	739,275
Reinsurance & Monoline Excess \| Casualty
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]
Earned premiums	174,630	184,903	346,349	366,670
Reinsurance & Monoline Excess \| Monoline excess
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]
Earned premiums	75,978	71,362	152,087	138,626
Reinsurance & Monoline Excess \| Property
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]
Earned premiums	$ 110,751	$ 113,136	$ 212,603	$ 233,979