Restructuring - Schedule of Restructuring and Related Expenses (Details) - 2017 Plan $ in Millions	3 Months Ended	9 Months Ended	21 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)
Restructuring Cost and Reserve [Line Items]
Costs incurred	$ 97	$ 366	$ 863
Estimated Remaining Costs	162	162	162
Expected total cost	1,025	1,025	1,025
Workforce reduction
Restructuring Cost and Reserve [Line Items]
Costs incurred	18	84	383
Estimated Remaining Costs	37	37	37
Expected total cost	420	420	420
Technology rationalization
Restructuring Cost and Reserve [Line Items]
Costs incurred	12	30	63
Estimated Remaining Costs	67	67	67
Expected total cost	130	130	130
Contract termination costs incurred	1	2	3
Expected contract termination cost remaining	15	15	15
Lease consolidation
Restructuring Cost and Reserve [Line Items]
Costs incurred	11	24	32
Estimated Remaining Costs	28	28	28
Expected total cost	60	60	60
Contract termination costs incurred	11	23	31
Expected contract termination cost remaining	80	80	80
Asset impairments
Restructuring Cost and Reserve [Line Items]
Costs incurred	2	11	37
Estimated Remaining Costs	3	3	3
Expected total cost	40	40	40
Other costs associated with restructuring and separation
Restructuring Cost and Reserve [Line Items]
Costs incurred	54	217	348
Estimated Remaining Costs	27	27	27
Expected total cost	375	375	375
Contract termination costs incurred	3	82	85
Expected contract termination cost remaining	$ 85	$ 85	$ 85