Statutory Financial Information (Actual Statutory Capital And Surplus) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
United States [Member]
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	$ 3,673,216	$ 3,158,828
United Kingdom [Member]
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	662,151	621,802
Bermuda [Member]
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	1,604,184	1,495,563
Other [Member]
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	$ 73,643	$ 74,704

The amount of statutory capital and surplus (stockholders' equity) as of the balance sheet date using prescribed or permitted statutory accounting practices (rather than GAAP, if different) of the state or country.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(23)(c))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6486098&loc=d3e24708-158529

Name:	us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant