Restructuring - Schedule of Restructuring and Related Expenses (Details) - 2017 Plan $ in Millions	3 Months Ended	6 Months Ended	18 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)
Restructuring Cost and Reserve [Line Items]
Costs incurred	$ 195	$ 269	$ 766
Estimated Remaining Costs	259	259	259
Expected total cost	1,025	1,025	1,025
Workforce reduction
Restructuring Cost and Reserve [Line Items]
Costs incurred	33	66	365
Estimated Remaining Costs	55	55	55
Expected total cost	420	420	420
Technology rationalization
Restructuring Cost and Reserve [Line Items]
Costs incurred	8	18	51
Estimated Remaining Costs	79	79	79
Expected total cost	130	130	130
Contract termination costs incurred	0	1	2
Expected contract termination cost remaining	15	15	15
Lease consolidation
Restructuring Cost and Reserve [Line Items]
Costs incurred	10	13	21
Estimated Remaining Costs	39	39	39
Expected total cost	60	60	60
Contract termination costs incurred	10	12	20
Expected contract termination cost remaining	80	80	80
Asset impairments
Restructuring Cost and Reserve [Line Items]
Costs incurred	8	9	35
Estimated Remaining Costs	5	5	5
Expected total cost	40	40	40
Other costs associated with restructuring and separation
Restructuring Cost and Reserve [Line Items]
Costs incurred	136	163	294
Estimated Remaining Costs	81	81	81
Expected total cost	375	375	375
Contract termination costs incurred	75	79	82
Expected contract termination cost remaining	$ 82	$ 82	$ 82