Fair Value Measurements (Details) - USD ($) $ in Millions		Oct. 31, 2015	Jul. 31, 2015
Liabilities:
Long-term debt		$ 500	$ 500
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement
Assets:
Total cash equivalents		262	695
Available-for-sale debt securities:
Total assets measured at fair value on a recurring basis		452	1,774
Liabilities:
Senior notes	[1]	530	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		262	695
Available-for-sale debt securities:
Total assets measured at fair value on a recurring basis		262	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]	530	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

[1] Carrying value on our balance sheets at October 31, 2015 was $500 million and at July 31, 2015 was $500 million. See Note 6.