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Summary of Significant Accounting Policies - Narrative (Details) - USD ($)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Summary of Significant Accounting Policies [Line Items]      
Property and equipment, net $ 26,529,000,000 $ 31,310,000,000  
Depreciation and amortization expense related to property and equipment 2,312,000,000 2,581,000,000 $ 2,329,000,000
Amortization of capitalized software 304,000,000 239,000,000 205,000,000
Net book value of capitalized software 1,000,000,000.0 1,100,000,000  
Fuel card obligation 1,100,000,000 736,000,000  
Advertising expense 119,000,000 $ 288,000,000 $ 267,000,000
Fuel Card Obligation      
Summary of Significant Accounting Policies [Line Items]      
Purchasing card maximum limit $ 1,100,000,000    
Minimum      
Summary of Significant Accounting Policies [Line Items]      
Estimated residual value (percent) 5.00%    
Maximum      
Summary of Significant Accounting Policies [Line Items]      
Estimated residual value (percent) 10.00%    
Software and software development costs | Minimum      
Summary of Significant Accounting Policies [Line Items]      
Estimated useful life 3 years    
Software and software development costs | Maximum      
Summary of Significant Accounting Policies [Line Items]      
Estimated useful life 10 years