Post-Employment Benefits - Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plans
Defined benefit plan net periodic benefit cost
Service cost - benefits earned during the year	$ 336	$ 250	$ 293
Interest cost on projected benefit obligations	300	337	308
Expected return on plans assets	(770)	(710)	(680)
Amortization of actuarial losses	255	132	205
Amortization of prior service cost (credits)	1	1	1
Total net cost	122	10	127
Net actuarial losses (gains), net of any prior service credits	611	944	86
Medical and Dental Plans
Defined benefit plan net periodic benefit cost
Service cost - benefits earned during the year	46	23	26
Interest cost on projected benefit obligations	42	52	48
Expected return on plans assets	(28)	(27)	(33)
Amortization of actuarial losses	21	22	33
Amortization of prior service cost (credits)	(28)	(32)	(45)
Total net cost	53	38	29
Net actuarial losses (gains), net of any prior service credits	$ (23)	$ 190	$ (53)