Employee Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Pension Benefits Cost
Components of expense/ (income) related to net benefit expense:
Service Cost Included in Labor and Fringe	$ 10	$ 10	$ 28	$ 30
Interest Cost	13	20	40	61
Expected Return on Plan Assets	(46)	(43)	(139)	(130)
Amortization of Net Loss	18	15	55	43
Total Included in Other Income - Net	(15)	(8)	(44)	(26)
Net Periodic Benefit (Credit)/Cost	(5)	2	(16)	4
Other Post-retirement Benefits Cost
Components of expense/ (income) related to net benefit expense:
Service Cost Included in Labor and Fringe	1	0	1	1
Interest Cost	0	1	1	2
Amortization of Prior Service Credits	(2)	(2)	(6)	(5)
Total Included in Other Income - Net	(2)	(1)	(5)	(3)
Net Periodic Benefit (Credit)/Cost	$ (1)	$ (1)	$ (4)	$ (2)