Income Taxes - Schedule of Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024
Deferred tax assets:
Accrued compensation and benefits	$ 250	$ 221
Accrued litigation	690	374
Client incentives	288	855
Net operating loss carryforwards	258	206
Comprehensive loss	206	79
Federal benefit of state taxes	74	16
Other	26	102
Valuation allowance	(264)	(212)
Total deferred tax assets	1,528	1,641
Deferred tax liabilities:
Property, equipment and technology, net	(296)	(295)
Intangible assets	(6,532)	(6,404)
Unrealized gains on equity securities	(71)	(81)
Foreign taxes	(50)	(22)
Total deferred tax liabilities	(6,949)	(6,802)
Net deferred tax liabilities	$ (5,421)	$ (5,161)