Employee Benefits - Net Periodic Benefit (Income) Expense (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Combined Pension Plan
Components of net periodic (benefit) expense
Service cost	$ 6	$ 6
Interest cost	63	68
Expected return on plan assets	(67)	(71)
Recognition of prior service credit	(2)	(2)
Recognition of actuarial (gain) loss	28	25
Net periodic post-retirement benefit expense	28	26
Post-Retirement Benefit Plans
Components of net periodic (benefit) expense
Service cost	1	1
Interest cost	23	26
Recognition of prior service credit	(2)	(2)
Recognition of actuarial (gain) loss	(4)	(5)
Net periodic post-retirement benefit expense	$ 18	$ 20