Pension and Postretirement Benefit Plans - Components of net periodic benefit cost and other information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net periodic benefit cost (benefit)
Net periodic benefit cost (benefit)	$ (73)	$ (128)	$ (196)
Funded | Postretirement Benefits
Net periodic benefit cost (benefit)
Service cost	52	52	54
Interest cost	79	80	79
Expected return on plan assets	(84)	(86)	(90)
Amortization of prior service benefit	(40)	(53)	(55)
Amortization of net actuarial loss	61	56	61
Settlements, curtailments, special terminations and other		(4)
Total non-operating expense (benefit)	16	(7)	(5)
Net periodic benefit cost (benefit)	68	45	49
Other changes in plan assets and benefit obligations recognized in other comprehensive (income) loss
Prior service cost (benefit)		(1)
Amortization of prior service benefit	40	53	55
Net actuarial (gain) loss	(127)	69	8
Amortization of net actuarial loss	(61)	(56)	(61)
Foreign currency	(2)
Total recognized in other comprehensive income (loss)	(150)	65	2
Total recognized in net periodic benefit cost (benefit) and other comprehensive (income) loss	(82)	110	51
Funded | United States | Qualified and Non-qualified Pension Benefits
Net periodic benefit cost (benefit)
Service cost	288	268	259
Interest cost	563	565	575
Expected return on plan assets	(1,087)	(1,035)	(1,043)
Amortization of prior service benefit	(23)	(23)	(24)
Amortization of net actuarial loss	503	388	354
Settlements, curtailments, special terminations and other		2	4
Total non-operating expense (benefit)	(44)	(103)	(134)
Net periodic benefit cost (benefit)	244	165	125
Other changes in plan assets and benefit obligations recognized in other comprehensive (income) loss
Prior service cost (benefit)			5
Amortization of prior service benefit	23	23	24
Net actuarial (gain) loss	(44)	605	692
Amortization of net actuarial loss	(503)	(388)	(354)
Total recognized in other comprehensive income (loss)	(524)	240	367
Total recognized in net periodic benefit cost (benefit) and other comprehensive (income) loss	(280)	405	492
Funded | International | Qualified and Non-qualified Pension Benefits
Net periodic benefit cost (benefit)
Service cost	143	142	133
Interest cost	157	157	171
Expected return on plan assets	(307)	(292)	(309)
Amortization of transition asset			(1)
Amortization of prior service benefit	(13)	(13)	(13)
Amortization of net actuarial loss	114	126	91
Settlements, curtailments, special terminations and other	4	4	4
Total non-operating expense (benefit)	(45)	(18)	(57)
Net periodic benefit cost (benefit)	98	124	76
Other changes in plan assets and benefit obligations recognized in other comprehensive (income) loss
Amortization of transition asset			1
Prior service cost (benefit)	7	6	(5)
Amortization of prior service benefit	13	13	13
Net actuarial (gain) loss	190	(248)	512
Amortization of net actuarial loss	(114)	(126)	(91)
Foreign currency	(83)	167	(93)
Total recognized in other comprehensive income (loss)	13	(188)	337
Total recognized in net periodic benefit cost (benefit) and other comprehensive (income) loss	$ 111	$ (64)	$ 413