Employee Benefits - Schedule of Net Periodic Benefit (Income) Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected return on plan assets	$ (686)	$ (668)
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	66	63	$ 64
Interest cost	392	409	425
Expected return on plan assets	(685)	(666)	(733)
Special termination benefits charge	15	0	13
Recognition of prior service credit	(8)	(8)	(8)
Recognition of actuarial loss	178	204	174
Net periodic pension benefit (income) expense	(42)	2	(65)
Other Postretirement Benefits Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	18	18	19
Interest cost	97	100	111
Expected return on plan assets	(1)	(2)	(7)
Special termination benefits charge	0	0	3
Recognition of prior service credit	20	20	20
Net periodic pension benefit (income) expense	$ 134	$ 136	$ 146