Restructuring - Narrative (Details) - 2017 Plan $ in Millions	3 Months Ended	6 Months Ended	18 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($) job_elimination	Jun. 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			3,337
Costs incurred	195	269	$ 766
Workforce reduction
Restructuring Cost and Reserve [Line Items]
Expected total cost	420	420	420
Costs incurred	33	66	365
Technology rationalization
Restructuring Cost and Reserve [Line Items]
Expected total cost	130	130	130
Costs incurred	8	18	51
Lease consolidation
Restructuring Cost and Reserve [Line Items]
Expected total cost	60	60	60
Costs incurred	10	13	21
Asset impairments
Restructuring Cost and Reserve [Line Items]
Expected total cost	40	40	40
Costs incurred	8	9	35
Other Restructuring
Restructuring Cost and Reserve [Line Items]
Expected total cost	375	375	375
Costs incurred	$ 136	$ 163	$ 294
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