Revenues - Accounts Receivable, Net (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Accounts Receivable, net	$ 1,326	$ 912
Freight Receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable, gross	950	716
Allowance for Credit Losses	(14)	(16)
Total Accounts Receivable, net	936	700
Non-Freight Receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable, gross	402	224
Allowance for Credit Losses	(12)	(12)
Total Accounts Receivable, net	$ 390	$ 212