Restructuring - Schedule of Restructuring and Related Expenses (Details) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2017 USD ($)
Restructuring Cost and Reserve [Line Items]
Estimated Remaining Costs	$ 349	$ 349
2017 Plan
Restructuring Cost and Reserve [Line Items]
Costs incurred	102	401
Expected total cost	750	750
Workforce reduction | 2017 Plan
Restructuring Cost and Reserve [Line Items]
Costs incurred	52	257
Estimated Remaining Costs	46	46
Expected total cost	303	303
Technology rationalization | 2017 Plan
Restructuring Cost and Reserve [Line Items]
Costs incurred	12	22
Estimated Remaining Costs	124	124
Expected total cost	146	146
Contract termination costs incurred	1	1
Expected contract termination cost remaining	10	10
Lease consolidation | 2017 Plan
Restructuring Cost and Reserve [Line Items]
Costs incurred	4	8
Estimated Remaining Costs	72	72
Expected total cost	80	80
Contract termination costs incurred	3	8
Expected contract termination cost remaining	80	80
Asset impairments | 2017 Plan
Restructuring Cost and Reserve [Line Items]
Costs incurred	2	26
Estimated Remaining Costs	14	14
Expected total cost	40	40
Other Restructuring | 2017 Plan
Restructuring Cost and Reserve [Line Items]
Costs incurred	32	88
Estimated Remaining Costs	93	93
Expected total cost	181	181
Contract termination costs incurred	1
Expected contract termination cost remaining	$ 10	$ 10