Restructuring - Schedule of Restructuring and Related Expenses (Details) $ in Millions	3 Months Ended	15 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2018 USD ($)
Restructuring Cost and Reserve [Line Items]
Estimated Remaining Costs	$ 454	$ 454
2017 Plan
Restructuring Cost and Reserve [Line Items]
Costs incurred	74	571
Expected total cost	1,025	1,025
Workforce reduction | 2017 Plan
Restructuring Cost and Reserve [Line Items]
Costs incurred	33	332
Estimated Remaining Costs	118	118
Expected total cost	450	450
Technology rationalization | 2017 Plan
Restructuring Cost and Reserve [Line Items]
Costs incurred	10	43
Estimated Remaining Costs	87	87
Expected total cost	130	130
Contract termination costs incurred	0	2
Expected contract termination cost remaining	15	15
Lease consolidation | 2017 Plan
Restructuring Cost and Reserve [Line Items]
Costs incurred	3	11
Estimated Remaining Costs	74	74
Expected total cost	85	85
Contract termination costs incurred	2	10
Expected contract termination cost remaining	80	80
Asset impairments | 2017 Plan
Restructuring Cost and Reserve [Line Items]
Costs incurred	1	27
Estimated Remaining Costs	23	23
Expected total cost	50	50
Other Restructuring | 2017 Plan
Restructuring Cost and Reserve [Line Items]
Costs incurred	27	158
Estimated Remaining Costs	152	152
Expected total cost	310	310
Contract termination costs incurred	4	7
Expected contract termination cost remaining	$ 80	$ 80