Casualty, Environmental and Other Reserves - Balance Sheet Presentation (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019		Dec. 31, 2018	Dec. 31, 2017
Contingencies [Line Items]
Current	$ 90	$ 100	[1]
Long-term	224	205	[1]
Total	314	305		$ 324	$ 374
Casualty
Contingencies [Line Items]
Current	49	48
Long-term	147	139
Total	196	187		199	228
Personal Injury
Contingencies [Line Items]
Current	38	42
Long-term	93	87
Total	131	129
Occupational
Contingencies [Line Items]
Current	11	6
Long-term	54	52
Total	65	58
Environmental
Contingencies [Line Items]
Current	23	31
Long-term	53	43
Total	76	74		80	90
Other
Contingencies [Line Items]
Current	18	21
Long-term	24	23
Total	$ 42	$ 44		$ 45	$ 56

[1] Certain prior year data has been reclassified to conform to the current presentation.