Summary of Significant Accounting Policies - Adoption of 606 (Details) - USD ($) $ in Millions	Sep. 30, 2019	Oct. 01, 2018	Sep. 30, 2018
Assets
Current portion of client incentives	$ 741	$ 539	$ 340
Client incentives	2,084	1,152	538
Liabilities
Client incentives	3,997	3,075	2,834
Accrued liabilities	1,625	1,166	1,160
Deferred tax liabilities	4,807	4,726	4,618
Other liabilities	2,939	2,724	2,666
Equity
Accumulated income	13,502	11,718	11,318
Fiscal Year 2018 Closing Balance Sheet
Assets
Current portion of client incentives	435		340
Client incentives	1,060		538
Liabilities
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
Cumulative Transition Adjustment for New Revenue Standard | Accounting Standards Update 2014-09
Assets
Current portion of client incentives	(306)	199
Client incentives	(1,024)	614
Liabilities
Client incentives	(498)	241
Accrued liabilities	(54)	6
Deferred tax liabilities	(141)	108
Other liabilities	(127)	58
Equity
Accumulated income	$ (538)	$ 400