Employee Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 46	$ 57
Interest cost	57	54
Expected return on plan assets	(72)	(73)
Amortization of unrecognized (gain) loss	29	33
Amortization of prior service credit	0	0
Total	60	71
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	3
Interest cost	15	15
Expected return on plan assets	0	0
Amortization of unrecognized (gain) loss	(15)	(8)
Amortization of prior service credit	(10)	(9)
Total	$ (8)	$ 1