Reserves For Losses And LAE (Reconciliation of the Net Incurred and Paid Claims Development) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for unpaid claims and claim adjustments expenses, net of reinsurance	$ 5,840,099
Total reinsurance recoverable on unpaid claims	4,215,348
Unallocated claims adjustment expenses	126,530
Other	27,542
Total other expenses	154,071
Total gross liability for unpaid claims and claim adjustment expense	10,209,519	$ 10,167,018	$ 9,343,028	$ 8,331,288
United States Reinsurance [Member] | Casualty Business [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for unpaid claims and claim adjustments expenses, net of reinsurance	1,611,666
Total reinsurance recoverable on unpaid claims	914,400
United States Reinsurance [Member] | Property Business [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for unpaid claims and claim adjustments expenses, net of reinsurance	1,035,477
Total reinsurance recoverable on unpaid claims	745,217
International [Member] | Casualty Business [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for unpaid claims and claim adjustments expenses, net of reinsurance	661,860
Total reinsurance recoverable on unpaid claims	364,868
International [Member] | Property Business [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for unpaid claims and claim adjustments expenses, net of reinsurance	997,937
Total reinsurance recoverable on unpaid claims	560,273
Insurance [Member] | Casualty Business [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for unpaid claims and claim adjustments expenses, net of reinsurance	1,382,275
Total reinsurance recoverable on unpaid claims	1,440,156
Insurance [Member] | Property Business [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for unpaid claims and claim adjustments expenses, net of reinsurance	150,884
Total reinsurance recoverable on unpaid claims	$ 190,435