Employee Benefit Plans - Net Periodic (Benefit) Cost (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Defined Benefit Plan Disclosure
Service cost		$ 0	$ 0	$ 0
Interest cost		700	833	781
Expected return on plan assets		(1,373)	(1,186)	(1,318)
Amortization of prior service credit		0	0	0
Recognized net actuarial loss		300	291	267
Settlements		38	5	4
Special termination benefits		0	0	0
Curtailment		0	0	0
Net periodic (benefit) cost		(335)	(57)	(266)
Other Postretirement and Postemployment Benefits
Defined Benefit Plan Disclosure
Service cost		96	83	85
Interest cost		120	137	126
Expected return on plan assets		(44)	(47)	(67)
Amortization of prior service credit		(9)	(9)	(24)
Recognized net actuarial loss		44	37	36
Settlements		0	0	0
Special termination benefits	$ 1,300	1,260	0	0
Curtailment		0	0	(53)
Net periodic (benefit) cost		$ 1,467	$ 201	$ 103