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Debt and Financing Activities - Commercial Paper (Details) - Commercial Paper - USD ($)
6 Months Ended
Sep. 30, 2017
Mar. 31, 2017
Debt Instrument [Line Items]    
Outstanding notes (up to) $ 3,500,000,000.0  
Borrowings from commercial paper 8,460,000,000  
Repayments made 8,343,000,000  
Commercial paper $ 300,000,000 $ 183,000,000
Weighted average interest rate (percent) 1.44% 1.20%