Restructuring - Narrative (Details) - 2017 Plan $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) job_elimination
Restructuring Cost and Reserve [Line Items]
Expected cost	$ 1,025
Number of positions eliminated | job_elimination	2,630
Incurred restructuring cost	$ 497
Workforce reduction
Restructuring Cost and Reserve [Line Items]
Expected cost	450
Incurred restructuring cost	299
Technology rationalization
Restructuring Cost and Reserve [Line Items]
Expected cost	130
Incurred restructuring cost	33
Lease consolidation
Restructuring Cost and Reserve [Line Items]
Expected cost	85
Incurred restructuring cost	8
Asset impairments
Restructuring Cost and Reserve [Line Items]
Expected cost	50
Incurred restructuring cost	26
Other Restructuring
Restructuring Cost and Reserve [Line Items]
Expected cost	310
Incurred restructuring cost	$ 131
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