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Debt and Credit Facilities - Additional Information (Details) - USD ($)
1 Months Ended
Nov. 30, 2018
Mar. 31, 2019
Dec. 31, 2018
364 Day Credit Agreement Expiring 2020 [Member]      
Line Of Credit Facility [Line Items]      
Initiation date Nov. 30, 2018    
Maximum borrowing capacity $ 5,000,000,000    
Three Year Agreement Expiring 2022 [Member]      
Line Of Credit Facility [Line Items]      
Initiation date Nov. 30, 2018    
Maximum borrowing capacity $ 5,000,000,000    
Five Year Agreement Expiring 2024 [Member]      
Line Of Credit Facility [Line Items]      
Initiation date Nov. 30, 2018    
Maximum borrowing capacity $ 5,000,000,000    
Total Committed Bank Credit Facilities [Member]      
Line Of Credit Facility [Line Items]      
Maximum borrowing capacity   $ 5,500,000,000  
Committed Bank Credit Facility Expiring 2020 [Member]      
Line Of Credit Facility [Line Items]      
Maximum borrowing capacity   2,800,000,000  
Committed Bank Credit Facility Expiring 2021 [Member]      
Line Of Credit Facility [Line Items]      
Maximum borrowing capacity   200,000,000  
Committed Bank Credit Facility Expiring 2022 [Member]      
Line Of Credit Facility [Line Items]      
Maximum borrowing capacity   $ 2,500,000,000  
Three Year Revolving Credit Facility Expiring 2022 [Member]      
Line Of Credit Facility [Line Items]      
Maximum borrowing capacity     $ 5,000,000,000
Credit facilities amount outstanding     $ 0