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Credit Facilities (Summary Of Outstanding Letters Of Credit And/Or Borrowings) (Details)
 9 Months Ended 12 Months Ended
Sep. 30, 2019
GBP (£)
Dec. 31, 2018
GBP (£)
Sep. 30, 2019
USD ($)
Dec. 31, 2018
USD ($)
May 26, 2016
USD ($)
Jun. 22, 2012
USD ($)
Line of Credit Facility [Line Items]            
Commitment £ 30,000,000   $ 1,000,000,000      
Wells Fargo Bank Group Credit Facility Tranche One [Member]            
Line of Credit Facility [Line Items]            
Commitment     200,000,000 $ 200,000,000    
In Use        
Wells Fargo Bank Group Credit Facility Tranche Two [Member]            
Line of Credit Facility [Line Items]            
Commitment     600,000,000 600,000,000    
In Use     595,575,000 558,818,000    
Date of Expiry Dec. 31, 2019 Dec. 31, 2019        
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     595,575,000 558,818,000    
Citibank Bilateral Letter Of Credit Agreement One [Member]            
Line of Credit Facility [Line Items]            
Commitment     200,000,000 200,000,000    
In Use     3,672,000 3,482,000    
Date of Expiry Nov. 24, 2019 Feb. 28, 2019        
Citibank Bilateral Letter Of Credit Agreement Two [Member]            
Line of Credit Facility [Line Items]            
In Use     74,243,000 3,672,000    
Date of Expiry Dec. 31, 2019 Nov. 24, 2019        
Citibank Bilateral Letter Of Credit Agreement Three [Member]            
Line of Credit Facility [Line Items]            
In Use     3,482,000 72,443,000    
Date of Expiry Feb. 29, 2020 Dec. 31, 2019        
Citibank Bilateral Letter Of Credit Agreement Four [Member]            
Line of Credit Facility [Line Items]            
In Use     173,000 296,000    
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     122,000 177,000    
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     2,993,000 125,000    
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     547,000 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     39,027,000 407,000    
Date of Expiry Sep. 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        
Citibank Bilateral Agreement [Member]            
Line of Credit Facility [Line Items]            
Commitment     200,000,000 200,000,000    
In Use     $ 124,259,000 $ 141,746,000    
Lloyds Bank PLC [Member]            
Line of Credit Facility [Line Items]            
Commitment | £ £ 30,000,000 £ 30,000,000        
In Use | £ £ 24,845,000 £ 26,000,000        
Date of Expiry Dec. 31, 2022 Dec. 31, 2022        
Lloyd's Bank Issuance One [Member]            
Line of Credit Facility [Line Items]            
Commitment | £        
In Use | £        
Lloyds Bank plc Credit Facility [Member]            
Line of Credit Facility [Line Items]            
Commitment | £ 30,000,000 30,000,000        
In Use | £ £ 24,845,000 £ 26,000,000