Employee Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pension Benefits Cost
Components of expense/ (income) related to net benefit expense:
Service Cost Included in Labor and Fringe	$ 10	$ 8	$ 30	$ 24
Interest Cost	20	25	61	77
Expected Return on Plan Assets	(43)	(43)	(130)	(129)
Amortization of Net Loss	15	8	43	23
Total Included in Other Income - Net	(8)	(10)	(26)	(29)
Net Periodic Benefit Cost/(Credit)	2	(2)	4	(5)
Other Post-retirement Benefits Cost
Components of expense/ (income) related to net benefit expense:
Service Cost Included in Labor and Fringe	0	0	1	1
Interest Cost	1	0	2	2
Amortization of Prior Service Costs	(2)	(2)	(5)	(5)
Total Included in Other Income - Net	(1)	(2)	(3)	(3)
Net Periodic Benefit Cost/(Credit)	$ (1)	$ (2)	$ (2)	$ (2)