Segment Reporting (Schedule Of Underwriting Results For Operating Segments) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Premiums earned	$ 1,375,623	$ 1,179,836	$ 2,646,077	$ 2,295,846
Incurred losses and LAE	843,222	1,228,760	1,639,318	1,942,015
Commission and brokerage	316,775	288,002	604,993	544,459
Other underwriting expenses	83,351	74,226	161,733	151,577
Underwriting gain (loss)	132,275	(411,152)	240,033	(342,205)
U.S. Reinsurance [Member]
Segment Reporting Information [Line Items]
Gross written premiums	641,763	652,110	1,405,903	1,296,332
Net written premiums	487,694	429,931	1,105,835	853,762
Premiums earned	605,136	467,509	1,180,040	908,894
Incurred losses and LAE	367,473	708,524	684,111	1,009,728
Commission and brokerage	172,589	148,711	339,692	276,031
Other underwriting expenses	15,728	15,472	31,319	32,358
Underwriting gain (loss)	49,346	(405,198)	124,918	(409,223)
International [Member]
Segment Reporting Information [Line Items]
Gross written premiums	372,671	398,714	761,642	765,738
Net written premiums	350,219	337,357	708,077	672,232
Premiums earned	353,086	343,133	675,460	672,072
Incurred losses and LAE	213,808	259,487	450,884	435,846
Commission and brokerage	85,974	84,379	155,836	162,773
Other underwriting expenses	9,632	9,869	18,097	19,955
Underwriting gain (loss)	43,672	(10,602)	50,643	53,498
Insurance [Member]
Segment Reporting Information [Line Items]
Gross written premiums	673,603	581,460	1,205,374	1,036,445
Net written premiums	481,952	423,066	898,196	771,387
Premiums earned	417,401	369,194	790,577	714,880
Incurred losses and LAE	261,941	260,749	504,323	496,441
Commission and brokerage	58,212	54,912	109,465	105,655
Other underwriting expenses	57,991	48,885	112,317	99,264
Underwriting gain (loss)	$ 39,257	$ 4,648	$ 64,472	$ 13,520