Employee Benefit Plans - Components of Net (Benefit) Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Benefits
Defined Benefit Plan Disclosure
Service cost	$ 0	$ 0
Interest cost	175	208
Expected return on plan assets	(343)	(297)
Amortization of prior service credit	0	0
Recognized net actuarial loss	75	73
Net periodic (benefit) cost	(93)	(16)
Other Postretirement and Postemployment Benefits
Defined Benefit Plan Disclosure
Service cost	24	21
Interest cost	28	34
Expected return on plan assets	(11)	(12)
Amortization of prior service credit	(2)	(2)
Recognized net actuarial loss	10	9
Net periodic (benefit) cost	$ 49	$ 50