Reserves For Losses And Loss Expenses (Reconciliation of Net Incurred and Paid Claims Development to Reserves for Loss and Loss Adjustment Expenses) (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	$ 14,665,237
Total due from reinsurers on unpaid claims	2,762,344
Total loss reserve discount	(416,358)
Gross reserves at end of year	17,011,223	$ 15,390,888	$ 13,784,430
Other liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	5,536,127
Workers' compensation
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	1,921,971
Professional liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	1,749,386
Commercial automobile
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	1,152,717
Short-tail lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	761,628
Casualty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	1,992,711
Monoline excess
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	1,146,311
Property
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	279,800
Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	11,246,415
Total due from reinsurers on unpaid claims	2,551,563
Total loss reserve discount	(12,491)
Insurance | Other liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	5,536,127
Total due from reinsurers on unpaid claims	730,029
Total loss reserve discount	0
Insurance | Workers' compensation
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	1,921,971
Total due from reinsurers on unpaid claims	221,769
Total loss reserve discount	(12,491)
Insurance | Professional liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	1,749,386
Total due from reinsurers on unpaid claims	1,019,810
Total loss reserve discount	0
Insurance | Commercial automobile
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	1,152,717
Total due from reinsurers on unpaid claims	67,895
Total loss reserve discount	0
Insurance | Short-tail lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	761,628
Total due from reinsurers on unpaid claims	414,549
Total loss reserve discount	0
Insurance | Other
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	124,586
Total due from reinsurers on unpaid claims	97,511
Total loss reserve discount	0
Reinsurance & Monoline Excess
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	3,418,822
Total due from reinsurers on unpaid claims	210,781
Total loss reserve discount	(403,867)
Reinsurance & Monoline Excess | Casualty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	1,992,711
Total due from reinsurers on unpaid claims	108,390
Total loss reserve discount	(84,668)
Reinsurance & Monoline Excess | Monoline excess
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	1,146,311
Total due from reinsurers on unpaid claims	35,926
Total loss reserve discount	319,199
Reinsurance & Monoline Excess | Property
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	279,800
Total due from reinsurers on unpaid claims	66,465
Total loss reserve discount	$ 0