Employee Benefits (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Combined Pension Plan
Components of net periodic (benefit) expense
Service cost	$ 15	$ 13	$ 28	$ 29
Interest cost	49	80	99	162
Expected return on plan assets	(138)	(149)	(276)	(298)
Recognition of prior service credit	(3)	(1)	(5)	(4)
Recognition of actuarial loss	49	52	98	102
Net periodic pension benefit (income) expense	(28)	(5)	(56)	(9)
Post-Retirement Benefit Plans
Components of net periodic (benefit) expense
Service cost	3	3	7	7
Interest cost	11	20	23	40
Recognition of prior service credit	4	4	8	8
Recognition of actuarial loss	1	0	2	0
Net periodic pension benefit (income) expense	$ 19	$ 27	$ 40	$ 55