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	$ 2,042,405	$ 1,817,299	$ 4,079,219	$ 3,549,996
Incurred losses and LAE	1,407,016	1,094,630	2,837,856	2,143,180
Commission and brokerage	466,316	420,950	914,838	810,424
Other underwriting expenses	118,130	104,833	246,990	203,818
Underwriting gain (loss)	50,943	196,886	79,535	392,574
U.S. Reinsurance [Member]
Segment Reporting Information [Line Items]
Gross written premiums	1,538,348	1,409,587	3,316,119	2,941,638
Net written premiums	1,424,089	1,234,686	3,037,183	2,629,239
Premiums earned	1,502,256	1,343,760	2,987,476	2,651,279
Incurred losses and LAE	1,005,677	783,082	2,026,319	1,555,280
Commission and brokerage	387,339	345,378	757,695	668,015
Other underwriting expenses	39,698	37,430	83,837	73,199
Underwriting gain (loss)	69,542	177,870	119,625	354,785
Insurance [Member]
Segment Reporting Information [Line Items]
Gross written premiums	830,990	757,068	1,624,090	1,352,125
Net written premiums	593,389	549,297	1,181,774	1,006,442
Premiums earned	540,149	473,539	1,091,743	898,717
Incurred losses and LAE	401,339	311,548	811,537	587,900
Commission and brokerage	78,977	75,572	157,143	142,409
Other underwriting expenses	78,432	67,403	163,153	130,619
Underwriting gain (loss)	$ (18,599)	$ 19,016	$ (40,090)	$ 37,789