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Debt and Financing Activities - Revolving Credit Facilities (Details)
3 Months Ended 6 Months Ended 12 Months Ended
Sep. 30, 2019
USD ($)
Sep. 30, 2019
USD ($)
Mar. 31, 2020
USD ($)
Mar. 31, 2018
USD ($)
Mar. 31, 2019
USD ($)
Global Facility          
Line of Credit Facility [Line Items]          
Credit facility borrowing capacity     $ 3,500,000,000    
Term of facility     5 years    
Amount borrowed under facility   $ 0   $ 0  
Credit facility outstanding         $ 0
Debt to capital ratio     0.65    
Line of Credit          
Line of Credit Facility [Line Items]          
Committed balance     $ 12,000,000    
Uncommitted balance     166,000,000    
Senior Unsecured Credit Facility (the 2020 Credit Facility) | Unsecured debt | Revolving Credit Facility          
Line of Credit Facility [Line Items]          
Credit facility borrowing capacity $ 4,000,000,000 4,000,000,000      
Term of facility 5 years        
Amount borrowed under facility     0    
Credit facility outstanding     $ 0    
Senior Unsecured Credit Facility (the 2020 Credit Facility), Canadian Dollar, British Pound Sterling, and Euros Sublimit | Unsecured debt | Revolving Credit Facility          
Line of Credit Facility [Line Items]          
Credit facility borrowing capacity $ 3,600,000,000 $ 3,600,000,000