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 |