Fair Value Measurements - Schedule of cash equivalents and available-for-sale debt securities by balance sheet classification (Details) - USD ($) $ in Millions	Jul. 31, 2019	Jul. 31, 2018
Available-for-sale debt securities:
Total available-for-sale debt securities	$ 824	$ 452
In cash and cash equivalents
Cash equivalents:
Total cash and cash equivalents	2,116	1,464
Fair Value, Measurements, Recurring
Cash equivalents:
Total cash and cash equivalents	1,647	1,143
Available-for-sale debt securities:
Total available-for-sale debt securities	824	452
Fair Value, Measurements, Recurring | Level 1
Cash equivalents:
Total cash and cash equivalents	1,647	1,143
Available-for-sale debt securities:
Total available-for-sale debt securities	0	0
Fair Value, Measurements, Recurring | Level 2
Cash equivalents:
Total cash and cash equivalents	0	0
Available-for-sale debt securities:
Total available-for-sale debt securities	824	452
Fair Value, Measurements, Recurring | Level 3
Cash equivalents:
Total cash and cash equivalents	0	0
Available-for-sale debt securities:
Total available-for-sale debt securities	0	0
Fair Value, Measurements, Recurring | In cash and cash equivalents
Cash equivalents:
Total cash and cash equivalents	1,647	1,143
Fair Value, Measurements, Recurring | In cash and cash equivalents | Level 1
Cash equivalents:
Total cash and cash equivalents	1,647	1,143
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 investments
Available-for-sale debt securities:
Total available-for-sale debt securities	624	252
Fair Value, Measurements, Recurring | In investments | Level 1
Available-for-sale debt securities:
Total available-for-sale debt securities	0	0
Fair Value, Measurements, Recurring | In investments | Level 2
Available-for-sale debt securities:
Total available-for-sale debt securities	624	252
Fair Value, Measurements, Recurring | In investments | Level 3
Available-for-sale debt securities:
Total available-for-sale debt securities	0	0
Fair Value, Measurements, Recurring | In funds held for customers
Available-for-sale debt securities:
Total available-for-sale debt securities	200	200
Fair Value, Measurements, Recurring | In funds held for customers | Level 1
Available-for-sale debt securities:
Total available-for-sale debt securities	0	0
Fair Value, Measurements, Recurring | In funds held for customers | Level 2
Available-for-sale debt securities:
Total available-for-sale debt securities	200	200
Fair Value, Measurements, Recurring | In funds held for customers | Level 3
Available-for-sale debt securities:
Total available-for-sale debt securities	$ 0	$ 0