Employee Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 57	$ 49	$ 114	$ 98
Interest cost	54	55	108	110
Expected return on plan assets	(73)	(61)	(146)	(121)
Amortization of unrecognized (gain) loss and prior service cost (credit)	32	32	65	63
Settlement loss	0	1	0	2
Total	70	76	141	152
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	3	4	6	6
Interest cost	15	17	30	34
Expected return on plan assets	0	(1)	0	(1)
Amortization of unrecognized (gain) loss and prior service cost (credit)	(18)	(18)	(35)	(36)
Settlement loss	0	0	0	0
Total	$ 0	$ 2	$ 1	$ 3