Employee Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Pension Benefits Cost
Components of expense/ (income) related to net benefit expense:
Service Cost Included in Labor and Fringe	$ 9	$ 10	$ 18	$ 20
Interest Cost	13	21	27	41
Expected Return on Plan Assets	(47)	(44)	(93)	(87)
Amortization of Net Loss	19	14	37	28
Total Included in Other Income - Net	(15)	(9)	(29)	(18)
Net Periodic Benefit (Credit)/Cost	(6)	1	(11)	2
Other Post-retirement Benefits Cost
Components of expense/ (income) related to net benefit expense:
Service Cost Included in Labor and Fringe	0	1	0	1
Interest Cost	1	0	1	1
Amortization of Prior Service Credits	(3)	(1)	(4)	(3)
Total Included in Other Income - Net	(2)	(1)	(3)	(2)
Net Periodic Benefit (Credit)/Cost	$ (2)	$ 0	$ (3)	$ (1)