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Debt and Financing Activities - Commercial Paper (Details) - USD ($)
12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Debt Instrument [Line Items]    
Commercial paper issuances $ 37,264,000,000 $ 20,542,000,000
Commercial paper repaid 37,264,000,000 20,725,000,000
Total debt outstanding 7,595,000,000 7,880,000,000
Commercial Paper    
Debt Instrument [Line Items]    
Credit facility borrowing capacity (up to) 3,500,000,000.0  
Total debt outstanding $ 0 $ 0