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 [Line Items]
Service cost	$ 60	$ 54
Interest cost	46	56
Expected return on plan assets	(71)	(91)
Amortization of unrecognized (gain) loss	43	35
Amortization of prior service credit	0	0
Special termination benefits	0	0
Other	0	3
Total	78	57
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	2
Interest cost	6	7
Expected return on plan assets	0	0
Amortization of unrecognized (gain) loss	(7)	(11)
Amortization of prior service credit	(31)	(31)
Special termination benefits	46	0
Other	0	0
Total	$ 16	$ (33)