Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jan. 31, 2018	Jan. 31, 2017	Jan. 31, 2018	Jan. 31, 2017	Jul. 31, 2018	Jul. 31, 2017
Income Tax [Line Items]
Federal statutory income tax rate	26.90%		26.90%	35.00%
Provisional income tax expense (benefit)	$ 39		$ 39
Excess tax benefits for provision for income taxes	8	$ 7	33	$ 26
Income tax provision (benefit)	40	(3)	(2)	(45)
Income (loss) before income taxes	19	$ 10	(40)	$ (62)
Provisional income tax expense	$ 4		$ 4
Effective income tax rate reconciliation, excluding discrete tax benefits, percent	27.00%	34.00%	27.00%	34.00%
Effective income tax rate				72.00%
Total amount of unrecognized tax benefits						$ 61
Unrecognized tax benefits, net of related deferred tax assets						38
Favorable net impact to income tax expense due to recognition of tax benefits						$ 38
Forecast
Income Tax [Line Items]
Federal statutory income tax rate					26.90%