Pension and Other Postretirement Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 57	$ 54	$ 80
Interest cost	81	82	110
Expected return on plan assets	(78)	(107)	(143)
Amortization of unrecognized prior service cost	(98)	(106)	(64)
Net loss amortization	1	3	5
Termination benefits	8	4	7
Curtailments	(31)	(18)	(19)
Settlements	0	0	0
Net periodic benefit cost (credit)	(60)	(88)	(24)
United States | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	312	282	307
Interest cost	454	456	434
Expected return on plan assets	(862)	(831)	(819)
Amortization of unrecognized prior service cost	(53)	(55)	(56)
Net loss amortization	180	119	214
Termination benefits	44	23	22
Curtailments	3	5	(12)
Settlements	0	0	1
Net periodic benefit cost (credit)	78	(1)	91
International | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	252	238	251
Interest cost	172	204	206
Expected return on plan assets	(393)	(382)	(379)
Amortization of unrecognized prior service cost	(11)	(11)	(14)
Net loss amortization	98	87	118
Termination benefits	4	4	1
Curtailments	(4)	(1)	(9)
Settlements	5	6	12
Net periodic benefit cost (credit)	$ 123	$ 145	$ 186