Fair Value Measurements (Details) - USD ($) $ in Millions		Jul. 31, 2016	Jul. 31, 2015
Available-for-sale securities:
Total available-for-sale securities		$ 1,412	$ 2,061
Liabilities:
Senior Notes		500	500
Municipal bonds
Available-for-sale securities:
Total available-for-sale securities		186	506
Municipal auction rate securities
Available-for-sale securities:
Total available-for-sale securities		15	15
Corporate notes
Available-for-sale securities:
Total available-for-sale securities		420	546
U.S. agency securities
Available-for-sale securities:
Total available-for-sale securities		36	12
Fair Value, Measurements, Recurring
Assets:
Cash equivalents, primarily money market funds		416	695
Available-for-sale securities:
Total available-for-sale securities		657	1,079
Total assets measured at fair value on a recurring basis		1,073	1,774
Liabilities:
Senior notes	[1]	515	531
Fair Value, Measurements, Recurring | Municipal bonds
Available-for-sale securities:
Total available-for-sale securities		186	506
Fair Value, Measurements, Recurring | Municipal auction rate securities
Available-for-sale securities:
Total available-for-sale securities		15	15
Fair Value, Measurements, Recurring | Corporate notes
Available-for-sale securities:
Total available-for-sale securities		420	546
Fair Value, Measurements, Recurring | U.S. agency securities
Available-for-sale securities:
Total available-for-sale securities		36	12
Fair Value, Measurements, Recurring | Level 1
Assets:
Cash equivalents, primarily money market funds		416	695
Available-for-sale securities:
Total available-for-sale securities		0	0
Total assets measured at fair value on a recurring basis		416	695
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		642	1,064
Total assets measured at fair value on a recurring basis		642	1,064
Liabilities:
Senior notes	[1]	515	531
Fair Value, Measurements, Recurring | Level 2 | Municipal bonds
Available-for-sale securities:
Total available-for-sale securities		186	506
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		420	546
Fair Value, Measurements, Recurring | Level 2 | U.S. agency securities
Available-for-sale securities:
Total available-for-sale securities		36	12
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	15
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 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	15
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, 2016 was $500 million and at July 31, 2015 was $500 million.