Revenues - Adoption of New Revenue Standard (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Oct. 01, 2018	Sep. 30, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenues	$ 5,840	$ 5,240	$ 16,840	$ 15,175
Operating Expenses
Marketing	282	240	799	724
Professional fees	113	112	305	312
General and administrative	315	230	855	688
Total operating expenses	1,932	2,355	5,574	5,627
Operating income	3,908	2,885	11,266	9,548
Income before income taxes	3,866	2,812	11,173	9,268
Income tax provision	765	483	2,118	1,812
Net income	3,101	$ 2,329	9,055	$ 7,456
Assets
Current portion of client incentives	690		690		$ 539	$ 340
Client incentives	1,854		1,854		1,152	538
Liabilities
Accounts payable	150		150			183
Client incentives	3,690		3,690		3,075	2,834
Accrued liabilities	1,358		1,358		1,166	1,160
Deferred tax liabilities	4,930		4,930		4,726	4,618
Other liabilities	2,801		2,801		2,724	2,666
Equity
Accumulated income	13,040		13,040		11,718	$ 11,318
Impact of the New Revenue Standard | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenues	(88)		(179)
Operating Expenses
Marketing	(31)		(100)
Professional fees	(5)		(12)
General and administrative	(11)		(21)
Total operating expenses	(47)		(133)
Operating income	(41)		(46)
Income before income taxes	(41)		(46)
Income tax provision	(8)		(6)
Net income	(33)		(40)
Assets
Current portion of client incentives	(282)		(282)		199
Client incentives	(811)		(811)		614
Liabilities
Accounts payable	26		26
Client incentives	(439)		(439)		241
Accrued liabilities	(18)		(18)		6
Deferred tax liabilities	(112)		(112)		108
Other liabilities	(110)		(110)		58
Equity
Accumulated income	(440)		(440)		$ 400
Results Under Prior Revenue Standard
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenues	5,752		16,661
Operating Expenses
Marketing	251		699
Professional fees	108		293
General and administrative	304		834
Total operating expenses	1,885		5,441
Operating income	3,867		11,220
Income before income taxes	3,825		11,127
Income tax provision	757		2,112
Net income	3,068		9,015
Assets
Current portion of client incentives	408		408
Client incentives	1,043		1,043
Liabilities
Accounts payable	176		176
Client incentives	3,251		3,251
Accrued liabilities	1,340		1,340
Deferred tax liabilities	4,818		4,818
Other liabilities	2,691		2,691
Equity
Accumulated income	$ 12,600		$ 12,600