Segment Reporting (Schedule Of Underwriting Results For Operating Segments) (Details) - USD ($) $ in Thousands	3 Months Ended					6 Months Ended		9 Months Ended	12 Months Ended
	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2017	Jun. 30, 2016	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Premiums earned	$ 468,197			$ 488,855		$ 939,252	$ 970,780
Incurred losses and LAE	297,228			338,536		586,950	634,598
Commission and brokerage	60,513	$ 52,507	$ 82,778	70,409	$ 66,365	113,020	136,774	$ 219,552	$ 281,424	$ 312,325	$ 338,975
Other underwriting expenses	64,197			58,330		124,092	117,557
Underwriting gain (loss)	46,259			21,580		115,190	81,851
U.S. Reinsurance [Member]
Segment Reporting Information [Line Items]
Gross written premiums	474,992			405,530		1,053,951	942,236
Net written premiums	135,177			161,031		354,239	384,458
Premiums earned	202,289			224,618		410,603	459,861
Incurred losses and LAE	119,747			110,152		240,181	226,322
Commission and brokerage	43,173			50,130		83,545	99,861
Other underwriting expenses	14,278			12,133		28,529	25,591
Underwriting gain (loss)	25,091			52,203		58,348	108,087
International [Member]
Segment Reporting Information [Line Items]
Gross written premiums	333,535			348,178		612,110	586,656
Net written premiums	111,961			124,438		215,207	212,154
Premiums earned	113,300			131,285		231,451	244,458
Incurred losses and LAE	78,459			91,427		146,873	164,842
Commission and brokerage	23,790			26,140		47,325	52,250
Other underwriting expenses	9,163			7,969		18,052	15,792
Underwriting gain (loss)	1,888			5,749		19,201	11,574
Insurance [Member]
Segment Reporting Information [Line Items]
Gross written premiums	527,047			424,083		921,898	778,803
Net written premiums	170,774			155,786		297,302	308,712
Premiums earned	152,608			132,952		297,198	266,461
Incurred losses and LAE	99,022			136,957		199,896	243,434
Commission and brokerage	(6,450)			(5,861)		(17,850)	(15,337)
Other underwriting expenses	40,756			38,228		77,511	76,174
Underwriting gain (loss)	$ 19,280			$ (36,372)		$ 37,641	$ (37,810)