Employee Benefit Plans - Balance Sheet Position (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Pension and Other Postretirement Defined Benefit Plans, Liabilities
Noncurrent liabilities	$ (10,630)	$ (8,452)
Pension Benefits
Pension and Other Postretirement Defined Benefit Plans, Liabilities
Current liabilities	(10)	(19)
Noncurrent liabilities	(6,075)	(5,335)
Total liabilities	(6,085)	(5,354)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, before Tax
Net actuarial loss	(9,878)	(8,765)
Prior service credit	0	0
Total accumulated other comprehensive loss, pre-tax	(9,878)	(8,765)
Other Postretirement and Postemployment Benefits
Pension and Other Postretirement Defined Benefit Plans, Liabilities
Current liabilities	(143)	(125)
Noncurrent liabilities	(4,127)	(2,647)
Total liabilities	(4,270)	(2,772)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, before Tax
Net actuarial loss	(886)	(715)
Prior service credit	29	38
Total accumulated other comprehensive loss, pre-tax	$ (857)	$ (677)