Revenues Adoption of New Revenue Standard (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018	Oct. 01, 2018	Sep. 30, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenues	$ 5,494	$ 5,073	$ 11,000	$ 9,935
Operating Expenses
Marketing	241	261	517	484
Professional fees	101	108	192	200
General and administrative	264	222	540	458
Total operating expenses	1,853	1,737	3,642	3,272
Operating income	3,641	3,336	7,358	6,663
Income before income taxes	3,677	3,217	7,307	6,456
Income tax provision	700	612	1,353	1,329
Net income	2,977	$ 2,605	5,954	$ 5,127
Assets
Current portion of client incentives	589		589			$ 340
Client incentives	1,664		1,664			538
Liabilities
Accounts payable	119		119			183
Client incentives	3,484		3,484			2,834
Accrued liabilities	1,207		1,207			1,160
Deferred tax liabilities	4,911		4,911			4,618
Other liabilities	2,669		2,669			2,666
Equity
Accumulated income	12,513		12,513			$ 11,318
Accounting Standards Update 2014-09
Assets
Current portion of client incentives					$ 539
Client incentives					1,152
Liabilities
Client incentives					3,075
Accrued liabilities					1,166
Deferred tax liabilities					4,726
Other liabilities					2,724
Equity
Accumulated income					11,718
Impact of the New Revenue Standard | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenues	(39)		(91)
Operating Expenses
Marketing	(39)		(69)
Professional fees	(3)		(6)
General and administrative	(7)		(10)
Total operating expenses	(49)		(85)
Operating income	10		(6)
Income before income taxes	10		(6)
Income tax provision	2		1
Net income	8		(7)
Assets
Current portion of client incentives	(262)		(262)		199
Client incentives	(725)		(725)		614
Liabilities
Accounts payable	13		13
Client incentives	(401)		(401)		241
Accrued liabilities	(8)		(8)		6
Deferred tax liabilities	(105)		(105)		108
Other liabilities	(79)		(79)		58
Equity
Accumulated income	(407)		(407)		$ 400
Results Under Prior Revenue Standard
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenues	5,455		10,909
Operating Expenses
Marketing	202		448
Professional fees	98		186
General and administrative	257		530
Total operating expenses	1,804		3,557
Operating income	3,651		7,352
Income before income taxes	3,687		7,301
Income tax provision	702		1,354
Net income	2,985		5,947
Assets
Current portion of client incentives	327		327
Client incentives	939		939
Liabilities
Accounts payable	132		132
Client incentives	3,083		3,083
Accrued liabilities	1,199		1,199
Deferred tax liabilities	4,806		4,806
Other liabilities	2,590		2,590
Equity
Accumulated income	$ 12,106		$ 12,106