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Debt and Financing Activities - Commercial Paper (Details) - USD ($)
12 Months Ended
Mar. 31, 2016
Mar. 31, 2017
Debt Instrument [Line Items]    
Commercial paper issuances $ 0  
Commercial Paper    
Debt Instrument [Line Items]    
Credit facility borrowing capacity (up to)   $ 3,500,000,000.0
Commercial paper outstanding $ 0 $ 183,000,000
Debt, Weighted Average Interest Rate   1.20%