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Credit Facilities (Summary Of Outstanding Letters Of Credit And Borrowings) (Details)
 12 Months Ended
Dec. 31, 2019
GBP (£)
Dec. 31, 2018
GBP (£)
Dec. 31, 2019
USD ($)
Nov. 07, 2019
GBP (£)
Dec. 31, 2018
USD ($)
May 26, 2016
USD ($)
Jun. 22, 2012
USD ($)
Line of Credit Facility [Line Items]              
Commitment £ 47,000,000   $ 1,000,000,000        
Wells Fargo Bank Group Credit Facility [Member]              
Line of Credit Facility [Line Items]              
Commitment     800,000,000   $ 800,000,000 $ 800,000,000 $ 800,000,000
In Use     623,484,000   558,818,000    
Wells Fargo Bank Group Credit Facility Tranche One [Member]              
Line of Credit Facility [Line Items]              
Commitment     200,000,000   200,000,000    
In Use     33,737,000      
Date of Expiry Dec. 31, 2020            
Wells Fargo Bank Group Credit Facility Tranche One [Member] | Debt Instrument Redemption Period One [Member]              
Line of Credit Facility [Line Items]              
In Use            
Wells Fargo Bank Group Credit Facility Tranche Two [Member]              
Line of Credit Facility [Line Items]              
Commitment     600,000,000   600,000,000    
In Use         558,818,000    
Date of Expiry   Dec. 31, 2019          
Wells Fargo Bank Group Credit Facility Tranche Two [Member] | Debt Instrument Redemption Period One [Member]              
Line of Credit Facility [Line Items]              
In Use     2,381,000        
Date of Expiry Jul. 29, 2020            
Wells Fargo Bank Group Credit Facility Tranche Two [Member] | Debt Instrument Redemption Period Two [Member]              
Line of Credit Facility [Line Items]              
In Use     1,649,000        
Date of Expiry Sep. 30, 2020            
Wells Fargo Bank Group Credit Facility Tranche Two [Member] | Debt Instrument Redemption Period Three [Member]              
Line of Credit Facility [Line Items]              
In Use     573,353,000        
Date of Expiry Dec. 31, 2020            
Wells Fargo Bank Group Credit Facility Tranche Two [Member] | Debt Instrument Redemption Period Four [Member]              
Line of Credit Facility [Line Items]              
Commitment     600,000,000        
In Use     12,364,000        
Date of Expiry Jan. 04, 2021            
Citibank Bilateral Agreement [Member]              
Line of Credit Facility [Line Items]              
Commitment     200,000,000   200,000,000    
In Use     147,970,000   141,746,000    
Citibank Bilateral Letter Of Credit Agreement One [Member]              
Line of Credit Facility [Line Items]              
Commitment     200,000,000   200,000,000    
In Use     4,425,000   3,482,000    
Date of Expiry Feb. 29, 2020 Feb. 28, 2019          
Citibank Bilateral Letter Of Credit Agreement Two [Member]              
Line of Credit Facility [Line Items]              
In Use     512,000   3,672,000    
Date of Expiry Sep. 03, 2020 Nov. 24, 2019          
Citibank Bilateral Letter Of Credit Agreement Three [Member]              
Line of Credit Facility [Line Items]              
In Use     3,672,000   72,443,000    
Date of Expiry Nov. 24, 2020 Dec. 31, 2019          
Citibank Bilateral Letter Of Credit Agreement Four [Member]              
Line of Credit Facility [Line Items]              
In Use     177,000   296,000    
Date of Expiry Dec. 16, 2020 Aug. 15, 2020          
Citibank Bilateral Letter Of Credit Agreement Five [Member]              
Line of Credit Facility [Line Items]              
In Use     125,000   177,000    
Date of Expiry Dec. 20, 2020 Dec. 16, 2020          
Citibank Bilateral Letter Of Credit Agreement Six [Member]              
Line of Credit Facility [Line Items]              
In Use     101,404,000   125,000    
Date of Expiry Dec. 31, 2020 Dec. 20, 2020          
Citibank Bilateral Letter Of Credit Agreement Seven [Member]              
Line of Credit Facility [Line Items]              
In Use     559,000   1,851,000    
Date of Expiry Aug. 15, 2021 Nov. 04, 2022          
Citibank Bilateral Letter Of Credit Agreement Eight [Member]              
Line of Credit Facility [Line Items]              
In Use     37,096,000   407,000    
Date of Expiry Dec. 30, 2023 Nov. 13, 2022          
Citibank Bilateral Letter Of Credit Agreement Nine [Member]              
Line of Credit Facility [Line Items]              
In Use       $ 59,293,000    
Date of Expiry   Dec. 30, 2022          
Lloyds Bank plc Credit Facility [Member]              
Line of Credit Facility [Line Items]              
Commitment | £ £ 47,000,000 £ 30,000,000   £ 47,000,000      
In Use | £ 47,000,000 26,000,000          
Lloyd's Bank Issuance One [Member]              
Line of Credit Facility [Line Items]              
Commitment | £          
In Use | £          
Lloyd S Bank Plc [Member]              
Line of Credit Facility [Line Items]              
Commitment | £ 47,000,000 30,000,000          
In Use | £ £ 47,000,000 £ 26,000,000          
Date of Expiry Dec. 31, 2023 Dec. 31, 2022