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Credit Facilities (Summary Of Outstanding Letters Of Credit And Borrowings) (Details)
 3 Months Ended 12 Months Ended
Mar. 31, 2021
GBP (£)
Dec. 31, 2020
GBP (£)
Mar. 31, 2021
USD ($)
Dec. 31, 2020
USD ($)
Mar. 25, 2020
USD ($)
May 26, 2016
USD ($)
Jun. 22, 2012
USD ($)
Line of Credit Facility [Line Items]              
Commitment £ 52,175,000   $ 1,050,000,000        
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     749,766,000 753,932,000      
Wells Fargo Bank Group Credit Facility Tranche One [Member]              
Line of Credit Facility [Line Items]              
Commitment     200,000,000        
In Use         $ 50,000,000    
Wells Fargo Bank Group Credit Facility Tranche One [Member] | Debt Instrument Redemption Period One [Member]              
Line of Credit Facility [Line Items]              
Commitment     200,000,000 200,000,000      
In Use     151,380,000 164,242,000      
Date of Expiry Dec. 31, 2021 Dec. 31, 2021          
Wells Fargo Bank Group Credit Facility Tranche Two [Member]              
Line of Credit Facility [Line Items]              
Commitment     600,000,000 600,000,000      
Wells Fargo Bank Group Credit Facility Tranche Two [Member] | Debt Instrument Redemption Period One [Member]              
Line of Credit Facility [Line Items]              
In Use     598,386,000 589,690,000      
Date of Expiry Dec. 31, 2021 Dec. 31, 2021          
Wells Fargo Bank Bilateral LOC Agreement [Member]              
Line of Credit Facility [Line Items]              
Commitment     50,000,000      
In Use     42,893,000      
Date of Expiry Dec. 31, 2021            
Citibank Bilateral Agreement [Member]              
Line of Credit Facility [Line Items]              
Commitment     200,000,000 200,000,000      
In Use     186,277,000 185,483,000      
Citibank Bilateral Letter Of Credit Agreement One [Member]              
Line of Credit Facility [Line Items]              
Commitment     200,000,000 200,000,000      
In Use     1,264,000 4,425,000      
Date of Expiry Nov. 24, 2021 Feb. 28, 2021          
Citibank Bilateral Letter Of Credit Agreement Two [Member]              
Line of Credit Facility [Line Items]              
In Use     452,000 3,672,000      
Date of Expiry Dec. 16, 2021 Nov. 24, 2021          
Citibank Bilateral Letter Of Credit Agreement Three [Member]              
Line of Credit Facility [Line Items]              
In Use     156,000 448,000      
Date of Expiry Dec. 20, 2021 Dec. 16, 2021          
Citibank Bilateral Letter Of Credit Agreement Four [Member]              
Line of Credit Facility [Line Items]              
In Use     139,005,000 115,000      
Date of Expiry Dec. 31, 2021 Dec. 20, 2021          
Citibank Bilateral Letter Of Credit Agreement Five [Member]              
Line of Credit Facility [Line Items]              
In Use     4,425,000 136,383,000      
Date of Expiry Feb. 28, 2022 Dec. 31, 2021          
Citibank Bilateral Letter Of Credit Agreement Six [Member]              
Line of Credit Facility [Line Items]              
In Use     828,000 39,619,000      
Date of Expiry Aug. 15, 2022 Dec. 30, 2024          
Citibank Bilateral Letter Of Credit Agreement Seven [Member]              
Line of Credit Facility [Line Items]              
In Use     40,147,000 821,000      
Date of Expiry Mar. 30, 2025 Aug. 15, 2022          
Lloyds Bank plc Credit Facility [Member]              
Line of Credit Facility [Line Items]              
Commitment | £ £ 52,175,000 £ 52,175,000          
In Use | £ £ 52,175,000 £ 52,175,000          
Lloyd's Bank Issuance One [Member]              
Line of Credit Facility [Line Items]              
Commitment     52,175,000 52,175,000      
In Use     $ 52,175,000 $ 52,175,000      
Date of Expiry Dec. 31, 2024 Dec. 31, 2023