Employee Benefit Plans - Components of Net (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
Service cost	$ 0	$ 0	$ 0	$ 0
Interest cost	175	208	526	625
Expected return on plan assets	(343)	(297)	(1,030)	(890)
Amortization of prior service credit	0	0	0	0
Recognized net actuarial loss	74	73	223	219
Special termination benefits	0	0	0	0
Settlements	30	2	33	3
Net periodic (benefit) cost	(64)	(14)	(248)	(43)
Other Postretirement and Postemployment Benefits
Defined Benefit Plan Disclosure
Service cost	24	21	72	63
Interest cost	30	34	86	102
Expected return on plan assets	(11)	(12)	(33)	(36)
Amortization of prior service credit	(2)	(2)	(7)	(7)
Recognized net actuarial loss	10	9	32	29
Special termination benefits	1,260	0	1,260	0
Settlements	0	0	0	0
Net periodic (benefit) cost	$ 1,311	$ 50	$ 1,410	$ 151