Pension and Postretirement Benefit Plans - Components of net periodic benefit cost and other information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net periodic benefit cost (benefit)
Net periodic benefit cost (benefit)	$ (50)	$ (68)	$ (73)
Funded | Postretirement Benefits
Net periodic benefit cost (benefit)
Service cost	43	43	52
Interest cost	62	82	79
Expected return on plan assets	(79)	(81)	(84)
Amortization of prior service benefit	(33)	(33)	(40)
Amortization of net actuarial loss	49	34	61
Settlements, curtailments, special terminations and other	3	5
Total non-operating expense (benefit)	2	7	16
Net periodic benefit cost (benefit)	45	50	68
Other changes in plan assets and benefit obligations recognized in other comprehensive (income) loss
Prior service cost (benefit)		(171)
Amortization of prior service benefit	33	33	40
Net actuarial (gain) loss	108	119	(127)
Amortization of net actuarial loss	(49)	(34)	(61)
Foreign currency	(7)	(1)	(2)
Settlements, curtailments, special termination benefits and other	(3)	(5)
Total recognized in other comprehensive income (loss)	82	(59)	(150)
Total recognized in net periodic benefit cost (benefit) and other comprehensive (income) loss	127	(9)	(82)
Funded | United States | Qualified and Non-qualified Pension Benefits
Net periodic benefit cost (benefit)
Service cost	261	251	288
Interest cost	499	620	563
Expected return on plan assets	(1,019)	(1,040)	(1,087)
Amortization of prior service benefit	(24)	(24)	(23)
Amortization of net actuarial loss	536	366	503
Settlements, curtailments, special terminations and other	16	70
Total non-operating expense (benefit)	8	(8)	(44)
Net periodic benefit cost (benefit)	269	243	244
Other changes in plan assets and benefit obligations recognized in other comprehensive (income) loss
Amortization of prior service benefit	24	24	23
Net actuarial (gain) loss	733	926	(44)
Amortization of net actuarial loss	(536)	(366)	(503)
Settlements, curtailments, special termination benefits and other	(16)	(35)
Total recognized in other comprehensive income (loss)	205	549	(524)
Total recognized in net periodic benefit cost (benefit) and other comprehensive (income) loss	474	792	(280)
Funded | International | Qualified and Non-qualified Pension Benefits
Net periodic benefit cost (benefit)
Service cost	152	131	143
Interest cost	117	156	157
Expected return on plan assets	(306)	(299)	(307)
Amortization of transition asset	2
Amortization of prior service benefit	(5)	(12)	(13)
Amortization of net actuarial loss	131	78	114
Settlements, curtailments, special terminations and other	1	10	4
Total non-operating expense (benefit)	(60)	(67)	(45)
Net periodic benefit cost (benefit)	92	64	98
Other changes in plan assets and benefit obligations recognized in other comprehensive (income) loss
Amortization of transition asset	(2)
Prior service cost (benefit)		3	7
Amortization of prior service benefit	5	12	13
Net actuarial (gain) loss	(358)	344	194
Amortization of net actuarial loss	(131)	(78)	(114)
Foreign currency	79	7	(83)
Settlements, curtailments, special termination benefits and other	(1)	(8)	(4)
Total recognized in other comprehensive income (loss)	(408)	280	13
Total recognized in net periodic benefit cost (benefit) and other comprehensive (income) loss	$ (316)	$ 344	$ 111