Fair Value Measurements (Details) - USD ($) $ in Millions		Jan. 31, 2016	Jul. 31, 2015
Liabilities:
Long-term debt		$ 1,000	$ 500
Fair value, measurements, recurring | Estimate of fair value measurement
Assets:
Total cash equivalents		280	695
Available-for-sale debt securities:
Total assets measured at fair value on a recurring basis		470	1,774
Liabilities:
Senior notes	[1]	523	531
Fair value, measurements, recurring | Estimate of fair value measurement | Municipal bonds
Available-for-sale debt securities:
Total available-for-sale securities		6	506
Fair value, measurements, recurring | Estimate of fair value measurement | Corporate notes
Available-for-sale debt securities:
Total available-for-sale securities		169	546
Fair value, measurements, recurring | Estimate of fair value measurement | U.S. agency securities
Available-for-sale debt securities:
Total available-for-sale securities		0	12
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		190	1,079
Fair value, measurements, recurring | Level 1
Assets:
Total cash equivalents		280	695
Available-for-sale debt securities:
Total assets measured at fair value on a recurring basis		280	695
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		175	1,064
Liabilities:
Senior notes	[1]	523	531
Fair value, measurements, recurring | Level 2 | Municipal bonds
Available-for-sale debt securities:
Total available-for-sale securities		6	506
Fair value, measurements, recurring | Level 2 | Corporate notes
Available-for-sale debt securities:
Total available-for-sale securities		169	546
Fair value, measurements, recurring | Level 2 | U.S. agency securities
Available-for-sale debt securities:
Total available-for-sale securities		0	12
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		175	1,064
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
5.75 percent fixed-rate notes due 2017
Liabilities:
Long-term debt		$ 500

[1] Carrying value in long-term debt on our balance sheets at January 31, 2016 was $500 million and at July 31, 2015 was $500 million. See Note 6, “Long-Term Obligations and Commitments,” for more information.