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Debt and Financing Activities - Commercial Paper (Details) - Commercial Paper - USD ($)
3 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Mar. 31, 2019
Debt Instrument [Line Items]      
Outstanding notes (up to) $ 3,500,000,000    
Proceeds from issuance of commercial paper 2,600,000,000 $ 9,000,000,000.0  
Repayments of commercial paper 2,600,000,000 $ 7,000,000,000.0  
Commercial paper $ 0   $ 0