Employee Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Pension Benefits Cost
Components of expense/ (income) related to net benefit expense:
Service Cost Included in Labor and Fringe	$ 9	$ 10
Interest Cost	14	20
Expected Return on Plan Assets	(46)	(43)
Amortization of Net Loss	18	14
Total Included in Other Income - Net	(14)	(9)
Net Periodic Benefit (Credit)/Cost	(5)	1
Other Post-retirement Benefits Cost
Components of expense/ (income) related to net benefit expense:
Service Cost Included in Labor and Fringe	0	0
Interest Cost	0	1
Amortization of Prior Service Credits	(1)	(2)
Total Included in Other Income - Net	(1)	(1)
Net Periodic Benefit (Credit)/Cost	$ (1)	$ (1)