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Debt and Financing Activities - Commercial Paper (Details) - USD ($)
12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Mar. 31, 2016
Debt Instrument [Line Items]      
Commercial paper issuances $ 20,542,000,000 $ 8,283,000,000 $ 0
Commercial paper repaid 20,725,000,000 8,100,000,000  
Commercial Paper      
Debt Instrument [Line Items]      
Credit facility borrowing capacity (up to) 3,500,000,000.0    
Commercial paper outstanding $ 0 $ 183,000,000  
Weighted average interest rate (percent)   1.20%