Employee Benefits (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Combined Pension Plan
Components of net periodic (benefit) expense
Service cost	$ 15	$ 14	$ 44	$ 42
Interest cost	81	109	243	326
Expected return on plan assets	(147)	(155)	(445)	(464)
Recognition of prior service credit	(3)	(2)	(7)	(6)
Recognition of actuarial loss	50	54	152	167
Net periodic pension benefit (income) expense	(4)	20	(13)	65
Post-Retirement Benefit Plans
Components of net periodic (benefit) expense
Service cost	4	3	11	11
Interest cost	17	27	57	82
Recognition of prior service credit	4	5	12	13
Curtailment loss	7	0	7	0
Net periodic pension benefit (income) expense	$ 32	$ 35	$ 87	$ 106