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, 2023	Dec. 31, 2022	Dec. 31, 2021
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	$ 16,051,744
Total due from reinsurers on unpaid claims	3,077,832
Total loss reserve discount	(389,924)
Gross reserves at end of year	18,739,652	$ 17,011,223	$ 15,390,888
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	6,371,675
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	1,812,508
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	1,971,299
Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	1,452,830
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	771,268
Casualty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	2,174,132
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	1,071,431
Property
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	275,397
Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	12,530,784
Total due from reinsurers on unpaid claims	2,867,620
Total loss reserve discount	(12,193)
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	6,371,675
Total due from reinsurers on unpaid claims	951,773
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	1,812,508
Total due from reinsurers on unpaid claims	189,182
Total loss reserve discount	(12,193)
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	1,971,299
Total due from reinsurers on unpaid claims	1,138,129
Total loss reserve discount	0
Insurance | Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	1,452,830
Total due from reinsurers on unpaid claims	81,334
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	771,268
Total due from reinsurers on unpaid claims	379,621
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	151,204
Total due from reinsurers on unpaid claims	127,581
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	3,520,960
Total due from reinsurers on unpaid claims	210,212
Total loss reserve discount	(377,731)
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	2,174,132
Total due from reinsurers on unpaid claims	116,198
Total loss reserve discount	(72,503)
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	1,071,431
Total due from reinsurers on unpaid claims	40,467
Total loss reserve discount	305,228
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	275,397
Total due from reinsurers on unpaid claims	53,547
Total loss reserve discount	$ 0