Employee Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 58	$ 46	$ 165	$ 138
Interest cost	52	56	164	170
Expected return on plan assets	(77)	(73)	(259)	(218)
Amortization of unrecognized (gain) loss	48	29	120	87
Amortization of prior service credit	0	0	0	0
Settlement loss	5	2	19	5
Settlement loss - VSPs (defined below)	319	0	390	0
Special termination benefit - VSPs	19	0	54	0
Curtailment	1	0	1	0
Total	425	60	654	182
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	3	2	8	7
Interest cost	7	10	21	39
Expected return on plan assets	0	0	(1)	(1)
Amortization of unrecognized (gain) loss	(9)	(12)	(31)	(42)
Amortization of prior service credit	(31)	(23)	(93)	(42)
Settlement loss	0	0	0	0
Settlement loss - VSPs (defined below)	0	0	0	0
Special termination benefit - VSPs	76	0	201	0
Curtailment	0	0	0	0
Total	$ 46	$ (23)	$ 105	$ (39)