Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Adoption of 606) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Oct. 01, 2018	Sep. 30, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Current portion of client incentives	$ 547		$ 340
Client incentives	1,264		538
Client incentives	3,345		2,834
Accrued liabilities	1,487		1,160
Deferred tax liabilities	4,835		4,618
Other liabilities	2,703		2,666
Accumulated income	11,908		$ 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	(198)	199
Client incentives	(661)	614
Client incentives	(260)	241
Accrued liabilities	(7)	6
Deferred tax liabilities	(109)	108
Other liabilities	(45)	58
Accumulated income	(415)	$ 400
Results Under Prior Revenue Standard
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Current portion of client incentives	349
Client incentives	603
Client incentives	3,085
Accrued liabilities	1,480
Deferred tax liabilities	4,726
Other liabilities	2,658
Accumulated income	$ 11,493