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Credit Facilities (Summary Of Outstanding Letters Of Credit And/Or Borrowings) (Details)
£ in Thousands, $ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2019
GBP (£)
Dec. 31, 2018
GBP (£)
Mar. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
May 26, 2016
USD ($)
Jun. 22, 2012
USD ($)
Line of Credit Facility [Line Items]            
Commitment £ 30,000   $ 1,000,000      
Wells Fargo Bank Group Credit Facility Tranche One [Member]            
Line of Credit Facility [Line Items]            
Commitment     200,000 $ 200,000    
In Use        
Wells Fargo Bank Group Credit Facility Tranche Two [Member]            
Line of Credit Facility [Line Items]            
Commitment     600,000 600,000    
In Use     587,989 558,818    
Date of Expiry Dec. 31, 2019 Dec. 31, 2019        
Wells Fargo Bank Group Credit Facility [Member]            
Line of Credit Facility [Line Items]            
Commitment     800,000 800,000 $ 800,000 $ 800,000
In Use     587,989 558,818    
Citibank Bilateral Letter Of Credit Agreement One [Member]            
Line of Credit Facility [Line Items]            
Commitment     200,000 200,000    
In Use     3,672 3,482    
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     72,491 3,672    
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 72,443    
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     298 296    
Date of Expiry Aug. 15, 2020 Aug. 15, 2020        
Citibank Bilateral Letter Of Credit Agreement Five [Member]            
Line of Credit Facility [Line Items]            
In Use     178 177    
Date of Expiry Dec. 16, 2020 Dec. 16, 2020        
Citibank Bilateral Letter Of Credit Agreement Six [Member]            
Line of Credit Facility [Line Items]            
In Use     126 125    
Date of Expiry Dec. 20, 2020 Dec. 20, 2020        
Citibank Bilateral Letter Of Credit Agreement Seven [Member]            
Line of Credit Facility [Line Items]            
In Use     3,082 1,851    
Date of Expiry Dec. 31, 2020 Nov. 04, 2022        
Citibank Bilateral Letter Of Credit Agreement Eight [Member]            
Line of Credit Facility [Line Items]            
In Use     42,528 407    
Date of Expiry Mar. 30, 2023 Nov. 13, 2022        
Citibank Bilateral Letter Of Credit Agreement Nine [Member]            
Line of Credit Facility [Line Items]            
In Use       59,293    
Date of Expiry   Dec. 30, 2022        
Citibank Bilateral Agreement [Member]            
Line of Credit Facility [Line Items]            
Commitment     200,000 200,000    
In Use     $ 125,857 $ 141,746    
Lloyds Bank PLC [Member]            
Line of Credit Facility [Line Items]            
Commitment | £ £ 30,000 £ 30,000        
In Use | £ £ 26,000 £ 26,000        
Date of Expiry Dec. 31, 2022 Dec. 31, 2022        
Lloyd's Bank Issuance One [Member]            
Line of Credit Facility [Line Items]            
Commitment | £        
Lloyds Bank plc Credit Facility [Member]            
Line of Credit Facility [Line Items]            
Commitment | £ 30,000 30,000        
In Use | £ £ 26,000 £ 26,000