Revenues - Schedule of Accounts Receivable (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Accounts Receivable, net	$ 1,036	$ 970
Freight Receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables gross	844	810
Allowance for Doubtful Accounts	(17)	(17)
Total Accounts Receivable, net	827	793
Non-Freight Receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables gross	218	186
Allowance for Doubtful Accounts	(9)	(9)
Total Accounts Receivable, net	$ 209	$ 177