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Financial Instruments (Narrative) (Details) - USD ($)
$ in Millions
9 Months Ended
Dec. 31, 2017
Mar. 31, 2017
May 31, 2016
Sale Leaseback Transaction [Line Items]      
Short-term Investments $ 173 $ 5,444  
Document Period End Date Dec. 31, 2017    
Marketable Securities, Equity Securities $ 60 46  
Time Deposits   3,000  
Commercial Paper   $ 2,400  
Handset Sale-Leaseback Tranche 2 [Member]      
Sale Leaseback Transaction [Line Items]      
Sale Leaseback Transaction, Net Book Value     $ 1,300
Repayments of Debt $ 385