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Reportable Segments Geographical information (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Segment Reporting Information [Line Items]      
Revenues $ 11,700 $ 13,209 $ 14,144
Long-term assets, excluding goodwill and other intangibles 46,466 46,792 48,299
U.S.      
Segment Reporting Information [Line Items]      
Revenues 11,625 12,833 13,596
Long-term assets, excluding goodwill and other intangibles 46,384 46,709 47,468
Canada      
Segment Reporting Information [Line Items]      
Revenues 0 300 447
Long-term assets, excluding goodwill and other intangibles 1 1 748
Mexico and other foreign      
Segment Reporting Information [Line Items]      
Revenues 75 76 101
Long-term assets, excluding goodwill and other intangibles $ 81 $ 82 $ 83