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Credit Facilities (Narrative) (Details)
£ in Thousands, $ in Thousands
 3 Months Ended 12 Months Ended
May 26, 2016
USD ($)
Jun. 22, 2012
USD ($)
Mar. 31, 2019
GBP (£)
item
Dec. 31, 2018
GBP (£)
Dec. 31, 2015
Mar. 31, 2019
USD ($)
item
Dec. 31, 2018
USD ($)
Line of Credit Facility [Line Items]              
Line of credit facility, number of facilities | item     2        
Line of credit facility, commitment     £ 30,000     $ 1,000,000  
Wells Fargo Bank Group Credit Facility [Member]              
Line of Credit Facility [Line Items]              
Line of credit facility, commitment $ 800,000 $ 800,000       $ 800,000 $ 800,000
Line of credit facility, term of credit facility 5 years 4 years          
Line of credit facility, number of tranches | item     2     2  
Line of credit facility, required minimum debt-to-capital ratio     0.35     0.35  
Line of credit facility, required minimum net worth           $ 5,370,979  
Percentage amount of consolidated net income required to be reserved to maintain compliance with facility covenants     25.00%        
Percentage amount of increase in consolidated net worth from new issuance required to be reserved to maintain compliance with facility covenants     25.00%        
Increase in reserves due to new issuance during the period required to maintain compliance with facility covenants           6,082,341  
Wells Fargo Bank Group Credit Facility Tranche One [Member]              
Line of Credit Facility [Line Items]              
Line of credit facility, commitment           200,000 200,000
Wells Fargo Bank Group Credit Facility Tranche One [Member] | Federal Funds [Member]              
Line of Credit Facility [Line Items]              
Line of credit facility, spread on variable rate basis     0.50%        
Wells Fargo Bank Group Credit Facility Tranche One [Member] | London Interbank Offered Rate (LIBOR) [Member]              
Line of Credit Facility [Line Items]              
Line of credit facility, spread on variable rate basis     1.00%        
Wells Fargo Bank Group Credit Facility Tranche Two [Member]              
Line of Credit Facility [Line Items]              
Line of credit facility, commitment           600,000 600,000
Line of Credit Facility, Expiration Date     Dec. 31, 2019 Dec. 31, 2019      
Citibank Bilateral Agreement [Member]              
Line of Credit Facility [Line Items]              
Line of credit facility, commitment           $ 200,000 $ 200,000
Percentage of interest on the principal amount of letters of credit issued     0.35%        
Percentage of interest on the principal amount of issued extended tenor letters of credit     0.45%        
Maximum term allowed to execute issued letters of credit     60 months        
Per annum commitment fee on undrawn credit     0.15%        
Lloyds Bank plc Credit Facility [Member]              
Line of Credit Facility [Line Items]              
Line of credit facility, commitment | £     £ 30,000 £ 30,000      
Line of credit facility, term of credit facility     4 years        
Line of credit facility, required minimum debt-to-capital ratio     0.35     0.35  
Line of credit facility, required minimum net worth           $ 5,326,009  
Consolidated net worth, percentage         70.00%    
Percentage amount of consolidated net income required to be reserved to maintain compliance with facility covenants     25.00%        
Percentage amount of increase in consolidated net worth from new issuance required to be reserved to maintain compliance with facility covenants     25.00%        
Increase in reserves due to new issuance during the period required to maintain compliance with facility covenants           $ 6,073,089  
Percentage commitment fee per annum     0.10%        
Per annum commitment fee on undrawn credit     0.35%