Revenues - Adoption of New Revenue Standard (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Oct. 01, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenues	$ 22,977	$ 20,609	$ 18,358
Operating Expenses
Marketing	1,105	988	922
Professional fees	454	446	409
General and administrative	1,196	1,145	1,060
Total operating expenses	7,976	7,655	6,214
Operating income	15,001	12,954	12,144
Income before income taxes	14,884	12,806	11,694
Income tax provision	2,804	2,505	4,995
Net income	12,080	10,301	$ 6,699
Assets
Current portion of client incentives	741	340		$ 539
Client incentives	2,084	538		1,152
Liabilities
Accounts payable	156	183
Client incentives	3,997	2,834		3,075
Accrued liabilities	1,625	1,160		1,166
Deferred tax liabilities	4,807	4,618		4,726
Other liabilities	2,939	2,666		2,724
Equity
Accumulated income	13,502	11,318		11,718
Impact of the New Revenue Standard | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenues	(352)
Operating Expenses
Marketing	(128)
Professional fees	(19)
General and administrative	(33)
Total operating expenses	(180)
Operating income	(172)
Income before income taxes	(172)
Income tax provision	(34)
Net income	(138)
Assets
Current portion of client incentives	(306)			199
Client incentives	(1,024)			614
Liabilities
Accounts payable	28
Client incentives	(498)			241
Accrued liabilities	(54)			6
Deferred tax liabilities	(141)			108
Other liabilities	(127)			58
Equity
Accumulated income	(538)			$ 400
Results Under Prior Revenue Standard
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenues	22,625
Operating Expenses
Marketing	977
Professional fees	435
General and administrative	1,163
Total operating expenses	7,796
Operating income	14,829
Income before income taxes	14,712
Income tax provision	2,770
Net income	11,942
Assets
Current portion of client incentives	435	340
Client incentives	1,060	538
Liabilities
Accounts payable	184
Client incentives	3,499	2,834
Accrued liabilities	1,571	1,160
Deferred tax liabilities	4,666	4,618
Other liabilities	2,812	2,666
Equity
Accumulated income	$ 12,964	$ 11,318