Post-Employment Benefits - Pretax Gains and Losses Included in Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined benefit plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarial loss (gain)	$ 701	$ 1,231	$ 209
Amortization of prior service credit (cost)	(2)	0	0
Amortization of actuarial loss	(227)	(109)	(140)
Foreign exchange loss (gain) and other	56	(6)	(13)
Total loss (gain)	528	1,116	56
Other post-employment plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarial loss (gain)	40	451	(287)
Amortization of prior service credit (cost)	4	0	0
Amortization of actuarial loss	(26)	(1)	(1)
Prior service cost (credit)	(397)	0	0
Total loss (gain)	$ (379)	$ 450	$ (288)