Fair Value Measurements - Schedule of cash equivalents and available-for-sale debt securities by balance sheet classification (Details) - USD ($) $ in Millions	Jul. 31, 2017	Jul. 31, 2016
In cash and cash equivalents
Cash equivalents:
Total cash and cash equivalents	$ 529	$ 638
Fair Value, Measurements, Recurring
Cash equivalents:
Total cash and cash equivalents	181	416
Available-for-sale securities:
Total available-for-sale securities	463	657
Fair Value, Measurements, Recurring | Level 1
Cash equivalents:
Total cash and cash equivalents	181	416
Available-for-sale securities:
Total available-for-sale securities	0	0
Fair Value, Measurements, Recurring | Level 2
Cash equivalents:
Total cash and cash equivalents	0	0
Available-for-sale securities:
Total available-for-sale securities	448	642
Fair Value, Measurements, Recurring | Level 3
Cash equivalents:
Total cash and cash equivalents	0	0
Available-for-sale securities:
Total available-for-sale securities	15	15
Fair Value, Measurements, Recurring | In cash and cash equivalents
Cash equivalents:
Total cash and cash equivalents	181	312
Fair Value, Measurements, Recurring | In cash and cash equivalents | Level 1
Cash equivalents:
Total cash and cash equivalents	181	312
Fair Value, Measurements, Recurring | In cash and cash equivalents | Level 2
Cash equivalents:
Total cash and cash equivalents	0	0
Fair Value, Measurements, Recurring | In cash and cash equivalents | Level 3
Cash equivalents:
Total cash and cash equivalents	0	0
Fair Value, Measurements, Recurring | In funds held for customers
Cash equivalents:
Total cash and cash equivalents	0	104
Fair Value, Measurements, Recurring | In funds held for customers | Level 1
Cash equivalents:
Total cash and cash equivalents	0	104
Fair Value, Measurements, Recurring | In funds held for customers | Level 2
Cash equivalents:
Total cash and cash equivalents	0	0
Fair Value, Measurements, Recurring | In funds held for customers | Level 3
Cash equivalents:
Total cash and cash equivalents	0	0
Fair Value, Measurements, Recurring | In investments
Available-for-sale securities:
Total available-for-sale securities	248	442
Fair Value, Measurements, Recurring | In investments | Level 1
Available-for-sale securities:
Total available-for-sale securities	0	0
Fair Value, Measurements, Recurring | In investments | Level 2
Available-for-sale securities:
Total available-for-sale securities	248	442
Fair Value, Measurements, Recurring | In investments | Level 3
Available-for-sale securities:
Total available-for-sale securities	0	0
Fair Value, Measurements, Recurring | In funds held for customers
Available-for-sale securities:
Total available-for-sale securities	200	200
Fair Value, Measurements, Recurring | In funds held for customers | Level 1
Available-for-sale securities:
Total available-for-sale securities	0	0
Fair Value, Measurements, Recurring | In funds held for customers | Level 2
Available-for-sale securities:
Total available-for-sale securities	200	200
Fair Value, Measurements, Recurring | In funds held for customers | Level 3
Available-for-sale securities:
Total available-for-sale securities	0	0
Fair Value, Measurements, Recurring | In long-term investments
Available-for-sale securities:
Total available-for-sale securities	15	15
Fair Value, Measurements, Recurring | In long-term investments | Level 1
Available-for-sale securities:
Total available-for-sale securities	0	0
Fair Value, Measurements, Recurring | In long-term investments | Level 2
Available-for-sale securities:
Total available-for-sale securities	0	0
Fair Value, Measurements, Recurring | In long-term investments | Level 3
Available-for-sale securities:
Total available-for-sale securities	$ 15	$ 15