Restructuring - Charges and Spending Relating to Restructuring Activities by Program (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning balance	$ 541	$ 645
Expenses	927	1,069	$ 1,110
(Payments) receipts, net	(722)	(760)
Non-cash activity	1	(413)
Restructuring reserve, ending balance	747	541	645
Separation Costs
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning balance	395	592
Expenses	552	216	208
(Payments) receipts, net	(328)	(413)
Non-cash activity	0	0
Restructuring reserve, ending balance	619	395	592
Accelerated Depreciation
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning balance	0	0
Expenses	60	227	174
(Payments) receipts, net	0	0
Non-cash activity	(60)	(227)
Restructuring reserve, ending balance	0	0	0
Other
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning balance	146	53
Expenses	315	626	728
(Payments) receipts, net	(394)	(347)
Non-cash activity	61	(186)
Restructuring reserve, ending balance	$ 128	$ 146	$ 53