Employee Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Defined Benefit Plan Disclosure [Line Items]
Nonoperating special termination benefits and settlement losses	$ 0	$ 415	$ 46	$ 646
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	59	58	179	165
Interest cost	46	52	138	164
Expected return on plan assets	(70)	(77)	(212)	(259)
Amortization of unrecognized (gain) loss	42	48	127	120
Amortization of prior service credit	0	0	0	0
Special termination benefits	0	19	0	54
Nonoperating special termination benefits and settlement losses	0	319	0	390
Other	2	6	3	20
Total	79	425	235	654
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	3	7	8
Interest cost	6	7	19	21
Expected return on plan assets	0	0	(1)	(1)
Amortization of unrecognized (gain) loss	(7)	(9)	(21)	(31)
Amortization of prior service credit	(30)	(31)	(92)	(93)
Special termination benefits	0	76	46	201
Nonoperating special termination benefits and settlement losses	0	0	0	0
Other	0	0	0	0
Total	$ (29)	$ 46	$ (42)	$ 105