Revenues Adoption of New Revenue Standard (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Oct. 01, 2018	Sep. 30, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenues	$ 5,506	$ 4,862
Operating Expenses
Marketing	276	223
Professional fees	91	92
General and administrative	276	236
Total operating expenses	1,789	1,535
Operating income	3,717	3,327
Income before income taxes	3,630	3,239
Income tax provision	653	717
Net income	2,977	$ 2,522
Assets
Current portion of client incentives	547			$ 340
Client incentives	1,264			538
Liabilities
Accounts payable	124			183
Client incentives	3,345			2,834
Other liabilities	2,703			2,666
Deferred tax liabilities	4,835			4,618
Accrued liabilities	1,487			1,160
Equity
Accumulated income	11,908			$ 11,318
Accounting Standards Update 2014-09
Assets
Current portion of client incentives			$ 539
Client incentives			1,152
Liabilities
Client incentives			3,075
Other liabilities			2,724
Deferred tax liabilities			4,726
Accrued liabilities			1,166
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	(52)
Operating Expenses
Marketing	(30)
Professional fees	(3)
General and administrative	(3)
Total operating expenses	(36)
Operating income	(16)
Income before income taxes	(16)
Income tax provision	(1)
Net income	(15)
Assets
Current portion of client incentives	(198)		199
Client incentives	(661)		614
Liabilities
Accounts payable	(23)
Client incentives	(260)		241
Other liabilities	(45)		58
Deferred tax liabilities	(109)		108
Accrued liabilities	(7)		6
Equity
Accumulated income	(415)		$ 400
Results Under Prior Revenue Standard
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenues	5,454
Operating Expenses
Marketing	246
Professional fees	88
General and administrative	273
Total operating expenses	1,753
Operating income	3,701
Income before income taxes	3,614
Income tax provision	652
Net income	2,962
Assets
Current portion of client incentives	349
Client incentives	603
Liabilities
Accounts payable	101
Client incentives	3,085
Other liabilities	2,658
Deferred tax liabilities	4,726
Accrued liabilities	1,480
Equity
Accumulated income	$ 11,493