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Debt and Financing Activities - Commercial Paper (Details) - Commercial Paper - USD ($)
6 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Mar. 31, 2021
Debt Instrument [Line Items]      
Outstanding notes (up to) $ 4,000,000,000    
Proceeds from issuance of commercial paper 3,000,000,000 $ 5,300,000,000  
Repayments of commercial paper 3,000,000,000 $ 5,300,000,000  
Commercial paper $ 0   $ 0