Employee Benefits - Schedule of Net Periodic Benefit (Income) Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected return on plan assets	$ (593)	$ (618)
Combined Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	59	56	$ 66
Interest cost	324	436	392
Combined Pension Plan | Qualified Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	59	56	66
Interest cost	324	436	392
Expected return on plan assets	(593)	(618)	(685)
Special termination benefits charge	13	6	15
Recognition of prior service credit	(9)	(8)	(8)
Recognition of actuarial loss	202	223	178
Net periodic pension benefit (income) expense	(4)	95	(42)
Post-Retirement Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	14	15	18
Interest cost	69	110	97
Expected return on plan assets	(1)	(1)	(1)
Recognition of prior service credit	16	16	20
Curtailment loss	8	0	0
Net periodic pension benefit (income) expense	$ 106	$ 140	$ 134