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Segment and Related Information - Geographical (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Revenues from External Customers and Long-Lived Assets [Line Items]      
Operating revenues $ 15,218 $ 15,455 $ 14,914
Property and equipment, net 14,148 12,893 11,942
US      
Revenues from External Customers and Long-Lived Assets [Line Items]      
Operating revenues 14,505 14,701 14,167
Property and equipment, net 13,168 11,941 11,044
Canada      
Revenues from External Customers and Long-Lived Assets [Line Items]      
Operating revenues 713 754 747
Property and equipment, net $ 980 $ 952 $ 898