Restructuring - Schedule of Restructuring and Related Expenses (Details) - 2017 Plan $ in Millions	3 Months Ended	27 Months Ended
	Mar. 31, 2019 USD ($)	Mar. 31, 2019 USD ($)
Restructuring Cost and Reserve [Line Items]
Costs incurred	$ 91	$ 1,073
Estimated Remaining Costs	152	152
Expected total cost	1,225	1,225
Workforce reduction
Restructuring Cost and Reserve [Line Items]
Costs incurred	24	438
Estimated Remaining Costs	12	12
Expected total cost	450	450
Technology rationalization
Restructuring Cost and Reserve [Line Items]
Costs incurred	11	91
Estimated Remaining Costs	39	39
Expected total cost	130	130
Contract termination costs incurred	1	7
Expected contract termination cost remaining	15	15
Lease consolidation
Restructuring Cost and Reserve [Line Items]
Costs incurred	9	45
Estimated Remaining Costs	20	20
Expected total cost	65	65
Contract termination costs incurred	9	42
Expected contract termination cost remaining	80	80
Asset impairments
Restructuring Cost and Reserve [Line Items]
Costs incurred	0	39
Estimated Remaining Costs	11	11
Expected total cost	50	50
Other costs associated with restructuring and separation
Restructuring Cost and Reserve [Line Items]
Costs incurred	47	460
Estimated Remaining Costs	70	70
Expected total cost	530	530
Contract termination costs incurred	2	90
Expected contract termination cost remaining	$ 95	$ 95