Employee Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 53	$ 46	$ 107	$ 92
Interest cost	56	57	112	114
Expected return on plan assets	(91)	(73)	(182)	(145)
Amortization of unrecognized (gain) loss	37	29	72	58
Amortization of prior service credit	0	0	0	0
Settlement loss	11	3	14	3
Settlement loss - VSPs (defined below)	71	0	71	0
Special termination benefit - VSPs	35	0	35	0
Total	172	62	229	122
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	3	3	5	5
Interest cost	7	14	14	29
Expected return on plan assets	(1)	(1)	(1)	(1)
Amortization of unrecognized (gain) loss	(11)	(15)	(22)	(30)
Amortization of prior service credit	(31)	(9)	(62)	(19)
Settlement loss	0	0	0	0
Settlement loss - VSPs (defined below)	0	0	0	0
Special termination benefit - VSPs	125	0	125	0
Total	$ 92	$ (8)	$ 59	$ (16)