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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 323,091	$ 430,224	$ 759,128	$ 862,133	$ 336,093	$ 369,642	$ 661,997	$ 643,578	$ 2,374,576	$ 2,011,310	$ 1,550,572
Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									0	(90)	309
Property
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									1,111,263	1,072,159	1,074,890
Property | U.S. and Caribbean
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									743,226	671,887	635,069
Property | Worldwide
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									210,168	234,801	210,441
Property | Worldwide (excluding U.S.)
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									55,043	76,370	137,466
Property | Japan
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									44,536	32,830	33,967
Property | Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									37,611	32,973	33,115
Property | Australia and New Zealand
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									13,729	15,869	22,746
Property | Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									6,950	7,429	2,086
Casualty and Specialty
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									1,263,313	939,241	475,373
Casualty and Specialty | U.S. and Caribbean
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									757,052	522,778	228,062
Casualty and Specialty | Worldwide
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									471,301	320,452	226,652
Casualty and Specialty | Worldwide (excluding U.S.)
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									13,840	87,597	6,946
Casualty and Specialty | Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									5,541	936	238
Casualty and Specialty | Australia and New Zealand
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									5,073	1,627	7,865
Casualty and Specialty | Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									$ 10,506	$ 5,851	$ 5,610