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Leases - Supplemental Cash Flows (Details) - USD ($)
$ in Millions
 3 Months Ended
Oct. 31, 2020
Oct. 31, 2019
Leases [Abstract]    
Cash paid for amounts included in the measurement of operating lease liabilities $ 14 $ 16
Right-of-use assets obtained in exchange for new operating lease liabilities 19 322
Right-of-use assets, obtained in exchange for new operating lease liabilities, existing prior to August 1, 2019   $ 319
Right-of-use assets, obtained in exchange for new operating lease liabilities, operating leases not yet commenced $ 3