Fair Value Measurements - Schedule of cash equivalents and available-for-sale debt securities by balance sheet classification (Details) - USD ($) $ in Millions	Jul. 31, 2020	Jul. 31, 2019
Available-for-sale debt securities:
Total available-for-sale debt securities	$ 808	$ 824
Level 3
Available-for-sale debt securities:
Loans held-for-sale	98
In cash and cash equivalents
Cash equivalents:
Total cash and cash equivalents	6,442	2,116
Fair Value, Measurements, Recurring
Cash equivalents:
Total cash and cash equivalents	5,765	1,647
Available-for-sale debt securities:
Total available-for-sale debt securities	808	824
Fair Value, Measurements, Recurring | Level 1
Cash equivalents:
Total cash and cash equivalents	5,765	1,647
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	808	824
Fair Value, Measurements, Recurring | In cash and cash equivalents
Cash equivalents:
Total cash and cash equivalents	5,765	1,647
Fair Value, Measurements, Recurring | In cash and cash equivalents | Level 1
Cash equivalents:
Total cash and cash equivalents	5,765	1,647
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 investments
Available-for-sale debt securities:
Total available-for-sale debt securities	608	624
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	608	624
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