Fair Value Measurements (Details) - USD ($) $ in Millions		Jul. 31, 2015	Jul. 31, 2014
Available-for-sale securities:
Total available-for-sale securities		$ 2,061	$ 2,234
Liabilities:
Senior notes, noncurrent		500	499
Senior Notes			499
Municipal bonds
Available-for-sale securities:
Total available-for-sale securities		506	701
Municipal auction rate securities
Available-for-sale securities:
Total available-for-sale securities		15	21
Corporate notes
Available-for-sale securities:
Total available-for-sale securities		546	466
U.S. agency securities
Available-for-sale securities:
Total available-for-sale securities		12	42
Fair Value, Measurements, Recurring
Assets:
Cash equivalents, primarily money market funds		695	652
Available-for-sale securities:
Total available-for-sale securities		1,079	1,230
Total assets measured at fair value on a recurring basis		1,774	1,882
Liabilities:
Senior notes	[1]	531	556
Fair Value, Measurements, Recurring | Municipal bonds
Available-for-sale securities:
Total available-for-sale securities		506	701
Fair Value, Measurements, Recurring | Municipal auction rate securities
Available-for-sale securities:
Total available-for-sale securities		15	21
Fair Value, Measurements, Recurring | Corporate notes
Available-for-sale securities:
Total available-for-sale securities		546	466
Fair Value, Measurements, Recurring | U.S. agency securities
Available-for-sale securities:
Total available-for-sale securities		12	42
Fair Value, Measurements, Recurring | Level 1
Assets:
Cash equivalents, primarily money market funds		695	652
Available-for-sale securities:
Total available-for-sale securities		0	0
Total assets measured at fair value on a recurring basis		695	652
Liabilities:
Senior notes	[1]	0	0
Fair Value, Measurements, Recurring | Level 1 | Municipal bonds
Available-for-sale securities:
Total available-for-sale securities		0	0
Fair Value, Measurements, Recurring | Level 1 | Municipal auction rate securities
Available-for-sale securities:
Total available-for-sale securities		0	0
Fair Value, Measurements, Recurring | Level 1 | Corporate notes
Available-for-sale securities:
Total available-for-sale securities		0	0
Fair Value, Measurements, Recurring | Level 1 | U.S. agency securities
Available-for-sale securities:
Total available-for-sale securities		0	0
Fair Value, Measurements, Recurring | Level 2
Assets:
Cash equivalents, primarily money market funds		0	0
Available-for-sale securities:
Total available-for-sale securities		1,064	1,209
Total assets measured at fair value on a recurring basis		1,064	1,209
Liabilities:
Senior notes	[1]	531	556
Fair Value, Measurements, Recurring | Level 2 | Municipal bonds
Available-for-sale securities:
Total available-for-sale securities		506	701
Fair Value, Measurements, Recurring | Level 2 | Municipal auction rate securities
Available-for-sale securities:
Total available-for-sale securities		0	0
Fair Value, Measurements, Recurring | Level 2 | Corporate notes
Available-for-sale securities:
Total available-for-sale securities		546	466
Fair Value, Measurements, Recurring | Level 2 | U.S. agency securities
Available-for-sale securities:
Total available-for-sale securities		12	42
Fair Value, Measurements, Recurring | Level 3
Assets:
Cash equivalents, primarily money market funds		0	0
Available-for-sale securities:
Total available-for-sale securities		15	21
Total assets measured at fair value on a recurring basis		15	21
Liabilities:
Senior notes	[1]	0	0
Fair Value, Measurements, Recurring | Level 3 | Municipal bonds
Available-for-sale securities:
Total available-for-sale securities		0	0
Fair Value, Measurements, Recurring | Level 3 | Municipal auction rate securities
Available-for-sale securities:
Total available-for-sale securities		15	21
Fair Value, Measurements, Recurring | Level 3 | Corporate notes
Available-for-sale securities:
Total available-for-sale securities		0	0
Fair Value, Measurements, Recurring | Level 3 | U.S. agency securities
Available-for-sale securities:
Total available-for-sale securities		$ 0	$ 0

[1] Carrying value on our balance sheets at July 31, 2015 was $500 million and at July 31, 2014 was $499 million. See Note 9.