Fair Value Measurements (Details) - USD ($) $ in Millions		Oct. 31, 2016	Jul. 31, 2016
Liabilities:
Long-term debt		$ 500	$ 500
Fair value, measurements, recurring | Estimate of fair value measurement
Assets:
Total cash equivalents		0	416
Available-for-sale debt securities:
Total assets measured at fair value on a recurring basis		460	1,073
Liabilities:
Senior notes	[1]	508	515
Fair value, measurements, recurring | Estimate of fair value measurement | Municipal bonds
Available-for-sale debt securities:
Total available-for-sale securities		89	186
Fair value, measurements, recurring | Estimate of fair value measurement | Corporate notes
Available-for-sale debt securities:
Total available-for-sale securities		356	420
Fair value, measurements, recurring | Estimate of fair value measurement | U.S. agency securities
Available-for-sale debt securities:
Total available-for-sale securities		0	36
Fair value, measurements, recurring | Estimate of fair value measurement | Municipal auction rate securities
Available-for-sale debt securities:
Total available-for-sale securities		15	15
Fair value, measurements, recurring | Estimate of fair value measurement | Available-for-sale corporate equity securities
Available-for-sale debt securities:
Total available-for-sale securities		460	657
Fair value, measurements, recurring | Level 1
Assets:
Total cash equivalents		0	416
Available-for-sale debt securities:
Total assets measured at fair value on a recurring basis		0	416
Liabilities:
Senior notes	[1]	0	0
Fair value, measurements, recurring | Level 1 | Municipal bonds
Available-for-sale debt securities:
Total available-for-sale securities		0	0
Fair value, measurements, recurring | Level 1 | Corporate notes
Available-for-sale debt securities:
Total available-for-sale securities		0	0
Fair value, measurements, recurring | Level 1 | U.S. agency securities
Available-for-sale debt securities:
Total available-for-sale securities		0	0
Fair value, measurements, recurring | Level 1 | Municipal auction rate securities
Available-for-sale debt securities:
Total available-for-sale securities		0	0
Fair value, measurements, recurring | Level 1 | Available-for-sale corporate equity securities
Available-for-sale debt securities:
Total available-for-sale securities		0	0
Fair value, measurements, recurring | Level 2
Assets:
Total cash equivalents		0	0
Available-for-sale debt securities:
Total assets measured at fair value on a recurring basis		445	642
Liabilities:
Senior notes	[1]	508	515
Fair value, measurements, recurring | Level 2 | Municipal bonds
Available-for-sale debt securities:
Total available-for-sale securities		89	186
Fair value, measurements, recurring | Level 2 | Corporate notes
Available-for-sale debt securities:
Total available-for-sale securities		356	420
Fair value, measurements, recurring | Level 2 | U.S. agency securities
Available-for-sale debt securities:
Total available-for-sale securities		0	36
Fair value, measurements, recurring | Level 2 | Municipal auction rate securities
Available-for-sale debt securities:
Total available-for-sale securities		0	0
Fair value, measurements, recurring | Level 2 | Available-for-sale corporate equity securities
Available-for-sale debt securities:
Total available-for-sale securities		445	642
Fair value, measurements, recurring | 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
Liabilities:
Senior notes	[1]	0	0
Fair value, measurements, recurring | Level 3 | Municipal bonds
Available-for-sale debt securities:
Total available-for-sale securities		0	0
Fair value, measurements, recurring | Level 3 | Corporate notes
Available-for-sale debt securities:
Total available-for-sale securities		0	0
Fair value, measurements, recurring | Level 3 | U.S. agency securities
Available-for-sale debt securities:
Total available-for-sale securities		0	0
Fair value, measurements, recurring | Level 3 | Municipal auction rate securities
Available-for-sale debt securities:
Total available-for-sale securities		15	15
Fair value, measurements, recurring | Level 3 | Available-for-sale corporate equity securities
Available-for-sale debt securities:
Total available-for-sale securities		$ 15	$ 15

[1] Carrying value on our balance sheets at October 31, 2016 was $500 million and at July 31, 2016 was $500 million. See Note 5, “Current Liabilities – Short-Term Debt,” for more information.