Restructuring - Schedule of Restructuring and Related Expense (Details) $ in Millions	12 Months Ended	24 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Restructuring Cost and Reserve [Line Items]
Estimated Remaining Costs	$ 243	$ 243
2017 Plan
Restructuring Cost and Reserve [Line Items]
Incurred restructuring cost	485	982
Expected cost	1,225	1,225
Workforce reduction | 2017 Plan
Restructuring Cost and Reserve [Line Items]
Incurred restructuring cost	115	414
Estimated Remaining Costs	36	36
Expected cost	450	450
Technology rationalization | 2017 Plan
Restructuring Cost and Reserve [Line Items]
Incurred restructuring cost	47	80
Estimated Remaining Costs	50	50
Expected cost	130	130
Incurred contract termination costs	5	6
Expected contract termination cost remaining	15	15
Lease consolidation | 2017 Plan
Restructuring Cost and Reserve [Line Items]
Incurred restructuring cost	28	36
Estimated Remaining Costs	29	29
Expected cost	65	65
Incurred contract termination costs	25	33
Expected contract termination cost remaining	80	80
Asset impairments | 2017 Plan
Restructuring Cost and Reserve [Line Items]
Incurred restructuring cost	13	39
Estimated Remaining Costs	11	11
Expected cost	50	50
Other Restructuring | 2017 Plan
Restructuring Cost and Reserve [Line Items]
Incurred restructuring cost	282	413
Estimated Remaining Costs	117	117
Expected cost	530	530
Incurred contract termination costs	85	88
Expected contract termination cost remaining	$ 95	$ 95