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Revenue - (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Revenue from Contract with Customer [Abstract]        
Revenue from contract liability $ 106 $ 216 $ 614 $ 660
Remaining revenue recognition $ 22,000   $ 22,000  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-10-01        
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction        
Remaining performance obligation, percentage 30.00%   30.00%  
Performance obligations expected to be satisfied, expected timing 12 months   12 months  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-10-01        
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction        
Remaining performance obligation, percentage 22.00%   22.00%  
Performance obligations expected to be satisfied, expected timing 13 months   13 months  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-10-01        
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction        
Performance obligations expected to be satisfied, expected timing 24 months   24 months