Pension and Other Postretirement Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 184	$ 228	$ 195
Interest cost	226	217	220
Expected return on plan assets	(291)	(292)	(243)
Amortization of unrecognized actuarial (gain) loss	118	130	128
Amortization of prior service credits	0	0	0
Other	5	1	5
Net periodic benefit cost (credit)	242	284	305
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	10	12	13
Interest cost	47	61	66
Expected return on plan assets	(1)	(2)	(2)
Amortization of unrecognized actuarial (gain) loss	(52)	(32)	(33)
Amortization of prior service credits	(73)	(37)	(37)
Other	0	0	0
Net periodic benefit cost (credit)	$ (69)	$ 2	$ 7