Segment Reporting Disclosures (Summary Of Segment Earned Premiums By Product) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Earned premiums	$ 3,823,532	$ 3,840,912	$ 3,231,616
U.S. Insurance [Member]
Segment Reporting Information [Line Items]
Earned premiums	2,105,212	2,022,860	1,727,766
U.S. Insurance [Member] | General Liability [Member]
Segment Reporting Information [Line Items]
Earned premiums	522,358	491,645	431,798
U.S. Insurance [Member] | Professional Liability [Member]
Segment Reporting Information [Line Items]
Earned premiums	324,230	321,005	268,203
U.S. Insurance [Member] | Property [Member]
Segment Reporting Information [Line Items]
Earned premiums	264,232	266,019	190,530
U.S. Insurance [Member] | Personal Lines [Member]
Segment Reporting Information [Line Items]
Earned premiums	325,811	299,442	185,935
U.S. Insurance [Member] | Programs [Member]
Segment Reporting Information [Line Items]
Earned premiums	277,829	244,216	205,004
U.S. Insurance [Member] | Workers' Compensation [Member]
Segment Reporting Information [Line Items]
Earned premiums	281,954	263,164	250,790
U.S. Insurance [Member] | Other Products [Member]
Segment Reporting Information [Line Items]
Earned premiums	108,798	137,369	195,506
International Insurance [Member]
Segment Reporting Information [Line Items]
Earned premiums	879,426	909,679	833,984
International Insurance [Member] | General Liability [Member]
Segment Reporting Information [Line Items]
Earned premiums	124,198	146,178	128,171
International Insurance [Member] | Professional Liability [Member]
Segment Reporting Information [Line Items]
Earned premiums	268,637	285,300	252,816
International Insurance [Member] | Property [Member]
Segment Reporting Information [Line Items]
Earned premiums	85,152	76,691	91,497
International Insurance [Member] | Marine And Energy [Member]
Segment Reporting Information [Line Items]
Earned premiums	262,307	287,263	287,745
International Insurance [Member] | Other Products [Member]
Segment Reporting Information [Line Items]
Earned premiums	139,132	114,247	73,755
Reinsurance [Member]
Segment Reporting Information [Line Items]
Earned premiums	838,543	908,385	669,826
Reinsurance [Member] | Property [Member]
Segment Reporting Information [Line Items]
Earned premiums	265,373	270,461	227,394
Reinsurance [Member] | Casualty [Member]
Segment Reporting Information [Line Items]
Earned premiums	315,027	323,390	244,981
Reinsurance [Member] | Auto [Member]
Segment Reporting Information [Line Items]
Earned premiums	102,227	152,645	84,042
Reinsurance [Member] | Other Products [Member]
Segment Reporting Information [Line Items]
Earned premiums	155,916	161,889	113,409
Other Insurance (Discontinued Lines) [Member]
Segment Reporting Information [Line Items]
Earned premiums	$ 351	$ (12)	$ 40