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, 2018	Dec. 31, 2017	Dec. 31, 2016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	$ 10,811,975
Total due from reinsurers on unpaid claims	1,717,565
Total loss reserve discount	(563,092)
Total gross reserves for loss and loss expenses	11,966,448	$ 11,670,408	$ 11,197,195
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	3,696,372
Primary 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	1,911,057
Excess 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	1,320,257
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	904,790
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	633,872
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	531,217
Casualty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	1,512,999
Property
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	189,632
Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	9,109,344
Total due from reinsurers on unpaid claims	1,551,282
Total loss reserve discount	(434,302)
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	3,696,372
Total due from reinsurers on unpaid claims	451,073
Total loss reserve discount	0
Insurance | Primary 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	1,911,057
Total due from reinsurers on unpaid claims	374,805
Total loss reserve discount	0
Insurance | Excess 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	1,320,257
Total due from reinsurers on unpaid claims	37,405
Total loss reserve discount	(434,302)
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	904,790
Total due from reinsurers on unpaid claims	344,958
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	633,872
Total due from reinsurers on unpaid claims	15,405
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	531,217
Total due from reinsurers on unpaid claims	293,376
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	111,779
Total due from reinsurers on unpaid claims	34,260
Total loss reserve discount	0
Reinsurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	1,702,631
Total due from reinsurers on unpaid claims	166,283
Total loss reserve discount	(128,790)
Reinsurance | Casualty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	1,512,999
Total due from reinsurers on unpaid claims	116,782
Total loss reserve discount	(128,790)
Reinsurance | Property
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total undiscounted reserves for loss and loss expenses, net of reinsurance	189,632
Total due from reinsurers on unpaid claims	49,501
Total loss reserve discount	$ 0