Fair Value Measurements - Schedule of financial assets and liabilities measured at fair value (Details) - USD ($) $ in Millions	Jul. 31, 2018	Jul. 31, 2017
Available-for-sale debt securities:
Total available-for-sale debt securities	$ 452	$ 463
Municipal bonds
Available-for-sale debt securities:
Total available-for-sale debt securities	31	63
Municipal auction rate securities
Available-for-sale debt securities:
Total available-for-sale debt securities	0	15
Corporate notes
Available-for-sale debt securities:
Total available-for-sale debt securities	412	382
U.S. agency securities
Available-for-sale debt securities:
Total available-for-sale debt securities	9	3
Fair Value, Measurements, Recurring
Assets:
Cash equivalents, primarily money market funds and savings deposit accounts	1,143	181
Available-for-sale debt securities:
Total available-for-sale debt securities	452	463
Total assets measured at fair value on a recurring basis	1,595	644
Fair Value, Measurements, Recurring | Municipal bonds
Available-for-sale debt securities:
Total available-for-sale debt securities	31	63
Fair Value, Measurements, Recurring | Municipal auction rate securities
Available-for-sale debt securities:
Total available-for-sale debt securities	0	15
Fair Value, Measurements, Recurring | Corporate notes
Available-for-sale debt securities:
Total available-for-sale debt securities	412	382
Fair Value, Measurements, Recurring | U.S. agency securities
Available-for-sale debt securities:
Total available-for-sale debt securities	9	3
Fair Value, Measurements, Recurring | Level 1
Assets:
Cash equivalents, primarily money market funds and savings deposit accounts	1,143	181
Available-for-sale debt securities:
Total available-for-sale debt securities	0	0
Total assets measured at fair value on a recurring basis	1,143	181
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 | Municipal auction rate securities
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 money market funds and savings deposit accounts	0	0
Available-for-sale debt securities:
Total available-for-sale debt securities	452	448
Total assets measured at fair value on a recurring basis	452	448
Fair Value, Measurements, Recurring | Level 2 | Municipal bonds
Available-for-sale debt securities:
Total available-for-sale debt securities	31	63
Fair Value, Measurements, Recurring | Level 2 | Municipal auction rate securities
Available-for-sale debt securities:
Total available-for-sale debt securities	0	0
Fair Value, Measurements, Recurring | Level 2 | Corporate notes
Available-for-sale debt securities:
Total available-for-sale debt securities	412	382
Fair Value, Measurements, Recurring | Level 2 | U.S. agency securities
Available-for-sale debt securities:
Total available-for-sale debt securities	9	3
Fair Value, Measurements, Recurring | Level 3
Assets:
Cash equivalents, primarily money market funds and savings deposit accounts	0	0
Available-for-sale debt securities:
Total available-for-sale debt securities	0	15
Total assets measured at fair value on a recurring basis	0	15
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 | Municipal auction rate securities
Available-for-sale debt securities:
Total available-for-sale debt securities	0	15
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