Restructuring - Schedule of Restructuring and Related Expenses (Details) $ in Millions	Mar. 31, 2017 USD ($)
Restructuring Cost and Reserve [Line Items]
Estimated Remaining Costs	$ 606
2017 Plan
Restructuring Cost and Reserve [Line Items]
First Quarter 2017	144
Expected total cost	750
Workforce reduction | 2017 Plan
Restructuring Cost and Reserve [Line Items]
First Quarter 2017	103
Estimated Remaining Costs	104
Expected total cost	207
Technology rationalization | 2017 Plan
Restructuring Cost and Reserve [Line Items]
First Quarter 2017	3
Estimated Remaining Costs	143
Expected total cost	146
Lease consolidation | 2017 Plan
Restructuring Cost and Reserve [Line Items]
First Quarter 2017	3
Estimated Remaining Costs	173
Expected total cost	176
Asset impairments | 2017 Plan
Restructuring Cost and Reserve [Line Items]
First Quarter 2017	13
Estimated Remaining Costs	27
Expected total cost	40
Other Restructuring | 2017 Plan
Restructuring Cost and Reserve [Line Items]
First Quarter 2017	22
Estimated Remaining Costs	159
Expected total cost	$ 181