Segment Reporting (Schedule Of Gross Written Premiums Derived From Largest Non-U.S. Market) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
United Kingdom [Member]
Segment Reporting Information [Line Items]
Gross written premium	$ 236,100	$ 207,140	$ 542,808	$ 470,997

Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums written.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 605
-Section 50
-Paragraph 1
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

Name:	us-gaap_DirectPremiumsWritten
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration