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Summary of Significant Accounting Policies - Property and equipment (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Property and equipment      
Accumulated depreciation on property and equipment $ 2,440 $ 2,810  
Depreciation expense on property and equipment $ 411 $ 353 $ 326
Equipment | Minimum      
Property and equipment      
Property and equipment, estimated useful life 3 years    
Equipment | Maximum      
Property and equipment      
Property and equipment, estimated useful life 10 years    
Real property      
Property and equipment      
Property and equipment, estimated useful life 40 years