Segment Reporting (Schedule Of Underwriting Results For Operating Segments) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Premiums earned	$ 1,538,960	$ 1,375,623	$ 3,032,965	$ 2,646,077
Incurred losses and LAE	975,832	843,222	2,005,345	1,639,318
Commission and brokerage	355,699	316,775	678,803	604,993
Other underwriting expenses	94,131	83,351	195,339	161,733
Underwriting gain (loss)	113,298	132,275	153,478	240,033
U.S. Reinsurance [Member]
Segment Reporting Information [Line Items]
Gross written premiums	1,124,895	1,014,434	2,433,089	2,167,545
Net written premiums	992,750	837,913	2,106,984	1,813,912
Premiums earned	1,062,833	958,222	2,069,868	1,855,500
Incurred losses and LAE	630,658	581,281	1,313,366	1,134,995
Commission and brokerage	292,286	258,563	553,187	495,528
Other underwriting expenses	26,599	25,360	56,446	49,416
Underwriting gain (loss)	113,290	93,018	146,869	175,561
Insurance [Member]
Segment Reporting Information [Line Items]
Gross written premiums	713,352	673,603	1,380,123	1,205,374
Net written premiums	508,801	481,952	1,033,275	898,196
Premiums earned	476,127	417,401	963,097	790,577
Incurred losses and LAE	345,174	261,941	691,979	504,323
Commission and brokerage	63,413	58,212	125,616	109,465
Other underwriting expenses	67,532	57,991	138,893	112,317
Underwriting gain (loss)	$ 8	$ 39,257	$ 6,609	$ 64,472