Employee Benefit Plans - Schedule of Net Benefit Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Components of expense/ (income) related to net benefit expense [Abstract]
Total (Income)/ Expense Included in Other Income - Net	$ (42)	$ (43)	$ (38)
Pension Benefits
Components of expense/ (income) related to net benefit expense [Abstract]
Service Cost Included in Labor and Fringe	43	31	32
Interest Cost	82	103	92
Expected Return on Plan Assets	(176)	(171)	(176)
Amortization of Net Loss	54	30	41
Amortization of Prior Service Cost	0	0	0
Total (Income)/ Expense Included in Other Income - Net	(40)	(38)	(43)
Net Periodic Benefit Cost/(Credit)	3	(7)	(11)
Settlement Gain	(1)	0	(1)
Total Periodic Benefit Cost/(Credit)	2	(7)	(12)
Post-retirement Benefits
Components of expense/ (income) related to net benefit expense [Abstract]
Service Cost Included in Labor and Fringe	1	1	2
Interest Cost	5	2	7
Expected Return on Plan Assets	0	0	0
Amortization of Net Loss	1	0	0
Amortization of Prior Service Cost	(7)	(7)	(2)
Total (Income)/ Expense Included in Other Income - Net	(1)	(5)	5
Net Periodic Benefit Cost/(Credit)	0	(4)	7
Settlement Gain	0	0	0
Total Periodic Benefit Cost/(Credit)	$ 0	$ (4)	$ 7