Fair Value Measurements - Schedule of financial assets and liabilities measured at fair value (Details) - USD ($) $ in Millions	Jul. 31, 2019	Jul. 31, 2018
Available-for-sale debt securities:
Total available-for-sale debt securities	$ 824	$ 452
Municipal bonds
Available-for-sale debt securities:
Total available-for-sale debt securities	5	31
Corporate notes
Available-for-sale debt securities:
Total available-for-sale debt securities	800	412
U.S. agency securities
Available-for-sale debt securities:
Total available-for-sale debt securities	19	9
Fair Value, Measurements, Recurring
Assets:
Cash equivalents, primarily time deposits, savings deposit accounts, and money market funds	1,647	1,143
Available-for-sale debt securities:
Total available-for-sale debt securities	824	452
Total assets measured at fair value on a recurring basis	2,471	1,595
Fair Value, Measurements, Recurring | Municipal bonds
Available-for-sale debt securities:
Total available-for-sale debt securities	5	31
Fair Value, Measurements, Recurring | Corporate notes
Available-for-sale debt securities:
Total available-for-sale debt securities	800	412
Fair Value, Measurements, Recurring | U.S. agency securities
Available-for-sale debt securities:
Total available-for-sale debt securities	19	9
Fair Value, Measurements, Recurring | Level 1
Assets:
Cash equivalents, primarily time deposits, savings deposit accounts, and money market funds	1,647	1,143
Available-for-sale debt securities:
Total available-for-sale debt securities	0	0
Total assets measured at fair value on a recurring basis	1,647	1,143
Fair Value, Measurements, Recurring | Level 1 | Municipal bonds
Available-for-sale debt securities:
Total available-for-sale debt securities	0	0
Fair Value, Measurements, Recurring | Level 1 | Corporate notes
Available-for-sale debt securities:
Total available-for-sale debt securities	0	0
Fair Value, Measurements, Recurring | Level 1 | U.S. agency securities
Available-for-sale debt securities:
Total available-for-sale debt securities	0	0
Fair Value, Measurements, Recurring | Level 2
Assets:
Cash equivalents, primarily time deposits, savings deposit accounts, and money market funds	0	0
Available-for-sale debt securities:
Total available-for-sale debt securities	824	452
Total assets measured at fair value on a recurring basis	824	452
Fair Value, Measurements, Recurring | Level 2 | Municipal bonds
Available-for-sale debt securities:
Total available-for-sale debt securities	5	31
Fair Value, Measurements, Recurring | Level 2 | Corporate notes
Available-for-sale debt securities:
Total available-for-sale debt securities	800	412
Fair Value, Measurements, Recurring | Level 2 | U.S. agency securities
Available-for-sale debt securities:
Total available-for-sale debt securities	19	9
Fair Value, Measurements, Recurring | Level 3
Assets:
Cash equivalents, primarily time deposits, savings deposit accounts, and money market funds	0	0
Available-for-sale debt securities:
Total available-for-sale debt securities	0	0
Total assets measured at fair value on a recurring basis	0	0
Fair Value, Measurements, Recurring | Level 3 | Municipal bonds
Available-for-sale debt securities:
Total available-for-sale debt securities	0	0
Fair Value, Measurements, Recurring | Level 3 | Corporate notes
Available-for-sale debt securities:
Total available-for-sale debt securities	0	0
Fair Value, Measurements, Recurring | Level 3 | U.S. agency securities
Available-for-sale debt securities:
Total available-for-sale debt securities	$ 0	$ 0