Fair Value Measurements - Fair Value Measurements on Recurring Basis (Details) - Fair value, measurements, recurring - USD ($) $ in Millions	Oct. 31, 2017	Jul. 31, 2017
Estimate of fair value measurement
Assets:
Total cash equivalents	$ 28	$ 181
Available-for-sale debt securities:
Total assets measured at fair value on a recurring basis	491	644
Estimate of fair value measurement | Municipal bonds
Available-for-sale debt securities:
Total available-for-sale securities	55	63
Estimate of fair value measurement | Corporate notes
Available-for-sale debt securities:
Total available-for-sale securities	383	382
Estimate of fair value measurement | U.S. agency securities
Available-for-sale debt securities:
Total available-for-sale securities	10	3
Estimate of fair value measurement | Municipal auction rate securities
Available-for-sale debt securities:
Total available-for-sale securities	15	15
Estimate of fair value measurement | Available-for-sale corporate equity securities
Available-for-sale debt securities:
Total available-for-sale securities	463	463
Level 1
Assets:
Total cash equivalents	28	181
Available-for-sale debt securities:
Total assets measured at fair value on a recurring basis	28	181
Level 1 | Municipal bonds
Available-for-sale debt securities:
Total available-for-sale securities	0	0
Level 1 | Corporate notes
Available-for-sale debt securities:
Total available-for-sale securities	0	0
Level 1 | U.S. agency securities
Available-for-sale debt securities:
Total available-for-sale securities	0	0
Level 1 | Municipal auction rate securities
Available-for-sale debt securities:
Total available-for-sale securities	0	0
Level 1 | Available-for-sale corporate equity securities
Available-for-sale debt securities:
Total available-for-sale securities	0	0
Level 2
Assets:
Total cash equivalents	0	0
Available-for-sale debt securities:
Total assets measured at fair value on a recurring basis	448	448
Level 2 | Municipal bonds
Available-for-sale debt securities:
Total available-for-sale securities	55	63
Level 2 | Corporate notes
Available-for-sale debt securities:
Total available-for-sale securities	383	382
Level 2 | U.S. agency securities
Available-for-sale debt securities:
Total available-for-sale securities	10	3
Level 2 | Municipal auction rate securities
Available-for-sale debt securities:
Total available-for-sale securities	0	0
Level 2 | Available-for-sale corporate equity securities
Available-for-sale debt securities:
Total available-for-sale securities	448	448
Level 3
Assets:
Total cash equivalents	0	0
Available-for-sale debt securities:
Total assets measured at fair value on a recurring basis	15	15
Level 3 | Municipal bonds
Available-for-sale debt securities:
Total available-for-sale securities	0	0
Level 3 | Corporate notes
Available-for-sale debt securities:
Total available-for-sale securities	0	0
Level 3 | U.S. agency securities
Available-for-sale debt securities:
Total available-for-sale securities	0	0
Level 3 | Municipal auction rate securities
Available-for-sale debt securities:
Total available-for-sale securities	15	15
Level 3 | Available-for-sale corporate equity securities
Available-for-sale debt securities:
Total available-for-sale securities	$ 15	$ 15