Segment Reporting (Schedule Of Gross Premiums Written Allocated To the Territory Of Coverage Exposure) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 407,766	$ 640,269	$ 827,415	$ 922,090	$ 323,091	$ 430,224	$ 759,128	$ 862,133	$ 2,797,540	$ 2,374,576	$ 2,011,310
Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									(7)	0	(90)
Property
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									1,440,437	1,111,263	1,072,159
Property | U.S. and Caribbean
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									954,269	743,226	671,887
Property | Worldwide
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									305,915	210,168	234,801
Property | Japan
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									49,821	44,536	32,830
Property | Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									49,486	37,611	32,973
Property | Worldwide (excluding U.S.)
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									48,182	55,043	76,370
Property | Australia and New Zealand
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									14,151	13,729	15,869
Property | Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									18,613	6,950	7,429
Casualty and Specialty
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									1,357,110	1,263,313	939,241
Casualty and Specialty | U.S. and Caribbean
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									622,757	646,381	522,778
Casualty and Specialty | Worldwide
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									686,253	581,972	320,452
Casualty and Specialty | Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									9,752	5,541	936
Casualty and Specialty | Worldwide (excluding U.S.)
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									10,104	13,840	87,597
Casualty and Specialty | Australia and New Zealand
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									4,141	5,073	1,627
Casualty and Specialty | Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									$ 24,103	$ 10,506	$ 5,851