Segment Reporting Disclosures (Summary Of Deferred Policy Acquisition Costs, Unearned Premiums And Unpaid Losses And Loss Adjustment Expenses) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	$ 566,042	$ 474,513
Unearned premiums	4,057,727	3,611,028
Unpaid losses and loss adjustment expenses	14,728,676	14,276,479	$ 13,584,281
Insurance [Member]
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	392,774	315,363
Unearned premiums	2,356,875	2,031,140
Unpaid losses and loss adjustment expenses	8,119,046	7,947,772
Reinsurance [Member]
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	173,268	159,150
Unearned premiums	677,260	630,435
Unpaid losses and loss adjustment expenses	3,395,459	3,425,751
Other Underwriting [Member]
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	0	0
Unearned premiums	0	0
Unpaid losses and loss adjustment expenses	298,062	386,329
Underwriting Operations [Member]
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	566,042	474,513
Unearned premiums	3,034,135	2,661,575
Unpaid losses and loss adjustment expenses	11,812,567	11,759,852
Program Services and Other [Member]
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	0	0
Unearned premiums	1,023,592	949,453
Unpaid losses and loss adjustment expenses	$ 2,916,109	$ 2,516,627