Summary of Significant Accounting Policies (Adoption of 606) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Oct. 01, 2018	Sep. 30, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Current portion of client incentives	$ 589		$ 340
Client incentives	1,664		538
Client incentives	3,484		2,834
Accrued liabilities	1,207		1,160
Deferred tax liabilities	4,911		4,618
Other liabilities	2,669		2,666
Accumulated income	12,513		$ 11,318
Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Current portion of client incentives		$ 539
Client incentives		1,152
Client incentives		3,075
Accrued liabilities		1,166
Deferred tax liabilities		4,726
Other liabilities		2,724
Accumulated income		11,718
Impact of the New Revenue Standard | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Current portion of client incentives	(262)	199
Client incentives	(725)	614
Client incentives	(401)	241
Accrued liabilities	(8)	6
Deferred tax liabilities	(105)	108
Other liabilities	(79)	58
Accumulated income	(407)	$ 400
Results Under Prior Revenue Standard
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Current portion of client incentives	327
Client incentives	939
Client incentives	3,083
Accrued liabilities	1,199
Deferred tax liabilities	4,806
Other liabilities	2,590
Accumulated income	$ 12,106