Revenues - Accounts Receivable (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Accounts Receivable, net	$ 957	$ 912
Freight Receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Trade Accounts Receivable, gross	786	716
Allowance for Credit Losses	(16)	(16)
Total Accounts Receivable, net	770	700
Non-Freight Receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Trade Accounts Receivable, gross	199	224
Allowance for Credit Losses	(12)	(12)
Total Accounts Receivable, net	$ 187	$ 212