Income Taxes - Deferred Income Taxes (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Compensation and benefits, assets	$ 554	$ 513
Loss and credit carryforwards, assets	1,900	1,327
Other, assets	501	396
Deferred income taxes, assets, gross	3,196	2,417
Valuation allowance, assets	(1,820)	(1,240)
Deferred income taxes, assets	1,376	1,177
Property and equipment, liabilities	361	312
Deferred Tax Liabilities, Other Finite-Lived Assets	2,408	2,624
Other, liabilities	137	74
Deferred income taxes, liabilities, gross	2,906	3,011
Deferred income taxes, liabilities	2,906	3,011
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Reserves	$ 241	$ 182