Employee Benefits (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Combined Pension Plan
Components of net periodic (benefit) expense
Service cost	$ 16	$ 14
Interest cost	82	110
Expected return on plan assets	(149)	(156)
Recognition of prior service credit	(3)	(2)
Recognition of actuarial loss	50	57
Net periodic pension benefit (income) expense	(4)	23
Post-Retirement Benefit Plans
Components of net periodic (benefit) expense
Service cost	4	4
Interest cost	20	27
Recognition of prior service credit	4	4
Net periodic pension benefit (income) expense	$ 28	$ 35