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, 2020	Dec. 31, 2019	Dec. 31, 2018
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	$ 12,103,860
Total due from reinsurers on unpaid claims	2,164,037
Total loss reserve discount	(483,467)
Gross reserves at end of year	13,784,430	$ 12,583,249	$ 11,966,448
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	4,436,965
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	1,843,751
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	1,203,104
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	835,375
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	631,217
Casualty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	1,644,886
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	1,233,027
Property
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	179,278
Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	9,046,669
Total due from reinsurers on unpaid claims	1,941,932
Total loss reserve discount	(11,700)
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	4,436,965
Total due from reinsurers on unpaid claims	609,006
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	1,843,751
Total due from reinsurers on unpaid claims	267,707
Total loss reserve discount	(11,700)
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	1,203,104
Total due from reinsurers on unpaid claims	644,841
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	835,375
Total due from reinsurers on unpaid claims	32,140
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	631,217
Total due from reinsurers on unpaid claims	352,168
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	96,257
Total due from reinsurers on unpaid claims	36,070
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	3,057,191
Total due from reinsurers on unpaid claims	222,105
Total loss reserve discount	(471,767)
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	1,644,886
Total due from reinsurers on unpaid claims	104,372
Total loss reserve discount	(100,536)
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	1,233,027
Total due from reinsurers on unpaid claims	41,598
Total loss reserve discount	371,231
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	179,278
Total due from reinsurers on unpaid claims	76,135
Total loss reserve discount	$ 0