Restructuring - Schedule of Restructuring and Related Expense (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Restructuring Cost and Reserve [Line Items]
Estimated Remaining Costs	$ 528
2017 Plan
Restructuring Cost and Reserve [Line Items]
Incurred restructuring cost	497
Expected cost	1,025
Workforce reduction | 2017 Plan
Restructuring Cost and Reserve [Line Items]
Incurred restructuring cost	299
Estimated Remaining Costs	151
Expected cost	450
Technology rationalization | 2017 Plan
Restructuring Cost and Reserve [Line Items]
Incurred restructuring cost	33
Estimated Remaining Costs	97
Expected cost	130
Incurred contract termination costs	1
Expected contract termination cost remaining	10
Lease consolidation | 2017 Plan
Restructuring Cost and Reserve [Line Items]
Incurred restructuring cost	8
Estimated Remaining Costs	77
Expected cost	85
Incurred contract termination costs	8
Expected contract termination cost remaining	80
Asset impairments | 2017 Plan
Restructuring Cost and Reserve [Line Items]
Incurred restructuring cost	26
Estimated Remaining Costs	24
Expected cost	50
Other Restructuring | 2017 Plan
Restructuring Cost and Reserve [Line Items]
Incurred restructuring cost	131
Estimated Remaining Costs	179
Expected cost	310
Incurred contract termination costs	3
Expected contract termination cost remaining	$ 10