Revenues - Accounts Receivable (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Accounts Receivable, net	$ 1,192	$ 912
Freight Receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable, gross	821	716
Allowance for Credit Losses	(14)	(16)
Total Accounts Receivable, net	807	700
Non-Freight Receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable, gross	397	224
Allowance for Credit Losses	(12)	(12)
Total Accounts Receivable, net	$ 385	$ 212