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Financial Instruments - Contractual Maturities of Investments in Available-for-Sale Debt Securities (Details)
$ in Millions
Apr. 30, 2018
USD ($)
Amortized Cost  
Due in one year $ 3
Due in more than five years 149
Debt securities, Amortized Cost 152
Fair Value  
Due in one year 3
Due in more than five years 170
Total $ 173