Segment Reporting Disclosures (Summary Of Deferred Policy Acquisition Costs, Unearned Premiums And Unpaid Losses And Loss Adjustment Expenses) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	$ 474,513	$ 465,569	$ 392,410	$ 352,756
Unearned premiums	3,611,028	3,308,779
Unpaid losses and loss adjustment expenses	14,276,479	13,584,281	$ 10,115,662
Insurance [Member]
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	315,363	286,780
Unearned premiums	2,031,140	1,855,331
Unpaid losses and loss adjustment expenses	7,947,772	7,711,510
Reinsurance [Member]
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	159,150	178,789
Unearned premiums	630,435	690,565
Unpaid losses and loss adjustment expenses	3,425,751	3,248,070
Other [Member]
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	0	0
Unearned premiums	0	0
Unpaid losses and loss adjustment expenses	386,329	429,270
Underwriting Operations [Member]
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	474,513	465,569
Unearned premiums	2,661,575	2,545,896
Unpaid losses and loss adjustment expenses	11,759,852	11,388,850
Program Services [Member]
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	0	0
Unearned premiums	949,453	762,883
Unpaid losses and loss adjustment expenses	$ 2,516,627	$ 2,195,431