Employee Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 54	$ 46
Interest cost	56	57
Expected return on plan assets	(91)	(72)
Amortization of unrecognized (gain) loss	35	29
Amortization of prior service credit	0	0
Settlement loss	3	0
Total	57	60
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	2
Interest cost	7	15
Expected return on plan assets	0	0
Amortization of unrecognized (gain) loss	(11)	(15)
Amortization of prior service credit	(31)	(10)
Settlement loss	0	0
Total	$ (33)	$ (8)