Employee Benefit Plans - Components of Net (Benefit) Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension Benefits
Defined Benefit Plan Disclosure
Service cost	$ 0	$ 0
Interest cost	208	195
Expected return on plan assets	(297)	(329)
Amortization of prior service credit	0	0
Recognized net actuarial loss	73	66
Net periodic (benefit) cost	(16)	(68)
Other Postretirement and Postemployment Benefits
Defined Benefit Plan Disclosure
Service cost	21	21
Interest cost	34	32
Expected return on plan assets	(12)	(17)
Amortization of prior service credit	(2)	(7)
Recognized net actuarial loss	9	10
Net periodic (benefit) cost	$ 50	$ 39