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Segment and Related Information - Geographical (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Revenues from External Customers and Long-Lived Assets [Line Items]      
Operating revenues $ 17,931 $ 15,218 $ 15,455
Property and equipment, net 14,419 14,148 12,893
US      
Revenues from External Customers and Long-Lived Assets [Line Items]      
Operating revenues 17,136 14,505 14,701
Property and equipment, net 13,428 13,168 11,941
Canada [Member]      
Revenues from External Customers and Long-Lived Assets [Line Items]      
Operating revenues 795 713 754
Property and equipment, net $ 991 $ 980 $ 952