Employee Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Defined Benefit Plan Disclosure [Line Items]
Settlement loss — Voluntary Programs (defined below)	$ 0	$ 231	$ 46	$ 231
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	60	53	120	107
Interest cost	46	56	92	112
Expected return on plan assets	(71)	(91)	(142)	(182)
Amortization of unrecognized (gain) loss	42	37	85	72
Amortization of prior service credit	0	0	0	0
Special termination benefits	0	35	0	35
Settlement loss — Voluntary Programs (defined below)	0	71	0	71
Other	1	11	1	14
Total	78	172	156	229
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	3	3	5	5
Interest cost	7	7	13	14
Expected return on plan assets	(1)	(1)	(1)	(1)
Amortization of unrecognized (gain) loss	(7)	(11)	(14)	(22)
Amortization of prior service credit	(31)	(31)	(62)	(62)
Special termination benefits	0	125	46	125
Settlement loss — Voluntary Programs (defined below)	0	0	0	0
Other	0	0	0	0
Total	$ (29)	$ 92	$ (13)	$ 59