Pension and Other Postretirement Plans - Components of Net Periodic Benefit Cost (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]
Curtailment gain	$ 0	$ 0	$ (107)
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	228	195	112
Interest cost	217	220	200
Expected return on plan assets	(292)	(243)	(216)
Curtailment gain	0	0	0
Amortization of unrecognized actuarial (gain) loss	130	128	76
Amortization of prior service credits	0	0	0
Other	1	5	5
Net periodic benefit cost (credit)	284	305	177
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	12	13	19
Interest cost	61	66	86
Expected return on plan assets	(2)	(2)	(2)
Curtailment gain	0	0	(107)
Amortization of unrecognized actuarial (gain) loss	(32)	(33)	(19)
Amortization of prior service credits	(37)	(37)	(31)
Other	0	0	0
Net periodic benefit cost (credit)	$ 2	$ 7	$ (54)