Employee Benefit Plans - Amounts Recognized in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Before tax basis amount in accumulated other comprehensive loss.
Total recognized at December 31	$ 7,278	$ 6,357
Other Benefits
Before tax basis amount in accumulated other comprehensive loss.
Net actuarial loss	260	74
Prior service (credit) costs	(186)	(228)
Total recognized at December 31	74	(154)
U.S. | Pension Benefits
Before tax basis amount in accumulated other comprehensive loss.
Net actuarial loss	5,714	5,135
Prior service (credit) costs	3	5
Total recognized at December 31	5,717	5,140
Int’l. | Pension Benefits
Before tax basis amount in accumulated other comprehensive loss.
Net actuarial loss	1,401	1,269
Prior service (credit) costs	86	102
Total recognized at December 31	$ 1,487	$ 1,371