Income Taxes - Summary of Changes in Valuation Allowance (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period	$ 6,258	$ 6,279	$ 5,906
Valuation allowance, deferred tax asset, increase (decrease), amount		48	121
Balance at end of period	6,256	6,258	6,279
Allowance provision for new operating and other loss carryforwards
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Valuation allowance, deferred tax asset, increase (decrease), amount	134	215	544
Allowance reductions
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Valuation allowance, deferred tax asset, increase (decrease), amount	$ (136)	$ (236)	$ (171)