Segment Reporting (Schedule Of Gross Written Premiums Derived From Largest Non-U.S. Market) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Gross written premiums	$ 3,217,999	$ 2,783,036	$ 2,240,951
United Kingdom [Member]
Segment Reporting Information [Line Items]
Gross written premiums	$ 1,116,363	$ 964,358	$ 914,612

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