Employee Benefit Plans - Components of Net Periodic (Benefit) Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Pension Benefits
Defined Benefit Plan Disclosure
Service cost	$ 0	$ 0
Interest cost	146	175
Expected return on plan assets	(375)	(343)
Amortization of prior service credit	0	0
Recognized net actuarial loss	89	75
Net periodic (benefit) cost	(140)	(93)
Other Postretirement and Postemployment Benefits
Defined Benefit Plan Disclosure
Service cost	21	24
Interest cost	29	28
Expected return on plan assets	(9)	(11)
Amortization of prior service credit	(2)	(2)
Recognized net actuarial loss	15	10
Net periodic (benefit) cost	$ 54	$ 49