Summary of Significant Accounting Policies (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) mi depreciable_asset_class	Dec. 31, 2019 USD ($)
Property, Plant and Equipment [Line Items]
Operating route miles \| mi	19,300
Employees covered by collective bargaining agreements	80.00%
Allowance for doubtful accounts \| $	$ 6	$ 9
Approximate number of depreciable asset classes used \| depreciable_asset_class	75
Equipment
Property, Plant and Equipment [Line Items]
Depreciation studies, period	3 years
Roadway
Property, Plant and Equipment [Line Items]
Depreciation studies, period	6 years
Chemicals
Property, Plant and Equipment [Line Items]
Market groups percent of total railway operating revenues	18.00%
Metals/construction
Property, Plant and Equipment [Line Items]
Market groups percent of total railway operating revenues	14.00%
Automotive
Property, Plant and Equipment [Line Items]
Market groups percent of total railway operating revenues	8.00%
Intermodal
Property, Plant and Equipment [Line Items]
Market groups percent of total railway operating revenues	27.00%
Agriculture, Forest And Consumer Products
Property, Plant and Equipment [Line Items]
Market groups percent of total railway operating revenues	22.00%
Coal
Property, Plant and Equipment [Line Items]
Market groups percent of total railway operating revenues	11.00%