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Revenue (Narrative) (Details) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2021
Dec. 31, 2020
Disaggregation of Revenue [Line Items]    
Advanced billings, revenue recognized $ 92  
Lease transactions $ 9 $ 8
Expected timing of satisfaction period 12 months  
Business services | Minimum    
Disaggregation of Revenue [Line Items]    
Period of recognition 1 year  
Business services | Maximum    
Disaggregation of Revenue [Line Items]    
Period of recognition 5 years