Restructuring - Narrative (Details) - 2017 Plan $ in Millions	3 Months Ended	9 Months Ended	21 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($) job_elimination	Sep. 30, 2018 USD ($) job_elimination
Restructuring Cost and Reserve [Line Items]
Expected total cost	$ 1,025	$ 1,025	$ 1,025
Number of positions eliminated to date | job_elimination			3,798
Costs incurred	97	366	$ 863
Workforce reduction
Restructuring Cost and Reserve [Line Items]
Expected total cost	420	420	420
Costs incurred	18	84	383
Technology rationalization
Restructuring Cost and Reserve [Line Items]
Expected total cost	130	130	130
Costs incurred	12	30	63
Lease consolidation
Restructuring Cost and Reserve [Line Items]
Expected total cost	60	60	60
Costs incurred	11	24	32
Asset impairments
Restructuring Cost and Reserve [Line Items]
Expected total cost	40	40	40
Costs incurred	2	11	37
Other Restructuring
Restructuring Cost and Reserve [Line Items]
Expected total cost	375	375	375
Costs incurred	$ 54	$ 217	$ 348
Minimum
Restructuring Cost and Reserve [Line Items]
Expected number of positions eliminated | job_elimination		4,200
Maximum
Restructuring Cost and Reserve [Line Items]
Expected number of positions eliminated | job_elimination		4,800