Employee Benefits (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Combined Pension Plan
Components of net periodic (benefit) expense
Service cost	$ 13	$ 16
Interest cost	50	82
Expected return on plan assets	(138)	(149)
Recognition of prior service credit	(2)	(3)
Recognition of actuarial loss	49	50
Net periodic pension benefit (income) expense	(28)	(4)
Post-Retirement Benefit Plans
Components of net periodic (benefit) expense
Service cost	4	4
Interest cost	12	20
Recognition of prior service credit	4	4
Recognition of actuarial loss	1	0
Net periodic pension benefit (income) expense	$ 21	$ 28