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Debt and Financing Activities - Commercial Paper (Details) - Commercial Paper - USD ($)
6 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Mar. 31, 2019
Debt Instrument [Line Items]      
Outstanding notes (up to) $ 4,000,000,000    
Proceeds from issuance of commercial paper 8,700,000,000 $ 19,700,000,000  
Repayments of commercial paper 8,100,000,000 $ 18,300,000,000  
Commercial paper $ 549,000,000   $ 0
Weighted average interest rate of notes outstanding (percent) 2.42%