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Reportable Segments Geographical information (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Segment Reporting Information [Line Items]      
Total Revenues $ 13,705 $ 13,058 $ 14,403
Long-term assets, excluding goodwill and other intangibles 51,079 51,606 53,939
U.S.      
Segment Reporting Information [Line Items]      
Total Revenues 13,073 12,459 13,797
Long-term assets, excluding goodwill and other intangibles 47,928 49,125 51,679
Canada      
Segment Reporting Information [Line Items]      
Total Revenues 503 483 479
Long-term assets, excluding goodwill and other intangibles 3,071 2,399 2,193
Mexico      
Segment Reporting Information [Line Items]      
Total Revenues 129 116 127
Long-term assets, excluding goodwill and other intangibles $ 80 $ 82 $ 67