Pension and Other Postretirement Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 216	$ 184	$ 228
Interest cost	209	226	217
Expected return on plan assets	(328)	(291)	(292)
Amortization of unrecognized actuarial (gain) loss	162	118	130
Amortization of prior service credits	0	0	0
Settlement loss - VSPs	430	0	0
Special termination benefit - VSPs	54	0	0
Curtailment	1	0	0
Other	(22)	(5)	(1)
Net periodic benefit cost (credit)	766	242	284
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	10	10	12
Interest cost	28	47	61
Expected return on plan assets	(1)	(1)	(2)
Amortization of unrecognized actuarial (gain) loss	(40)	(52)	(32)
Amortization of prior service credits	(124)	(73)	(37)
Settlement loss - VSPs	0	0	0
Special termination benefit - VSPs	201	0	0
Curtailment	0	0	0
Other	0	0	0
Net periodic benefit cost (credit)	$ 74	$ (69)	$ 2