Revenues - Accounts Receivable (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Accounts Receivable, net	$ 1,106	$ 1,010
Freight Receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable, gross	891	846
Allowance for Doubtful Accounts	(17)	(18)
Total Accounts Receivable, net	874	828
Non-Freight Receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable, gross	241	190
Allowance for Doubtful Accounts	(9)	(8)
Total Accounts Receivable, net	$ 232	$ 182