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Debt and Financing Activities - Commercial Paper (Details) - Commercial Paper - USD ($)
6 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Mar. 31, 2020
Debt Instrument [Line Items]      
Outstanding notes (up to) $ 4,000,000,000.0    
Proceeds from issuance of commercial paper 5,300,000,000 $ 8,700,000,000  
Repayments of commercial paper 5,300,000,000 $ 8,100,000,000  
Commercial paper $ 0   $ 0