Revenues - Receivables (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Accounts Receivable, net	$ 1,010	$ 970
Freight Receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross Receivables	846	810
Allowance for Doubtful Accounts	(18)	(17)
Total Accounts Receivable, net	828	793
Non-Freight Receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross Receivables	190	186
Allowance for Doubtful Accounts	(8)	(9)
Total Accounts Receivable, net	$ 182	$ 177