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Revenue - (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Revenue from Contract with Customer [Abstract]        
Revenue from contract liability $ 128 $ 178 $ 636 $ 629
Remaining revenue recognition $ 20,000   $ 20,000  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-10-01        
Revenue from Contract with Customer [Abstract]        
Remaining performance obligation, percentage 35.00%   35.00%  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction        
Performance obligations expected to be satisfied, expected timing 12 months   12 months  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-10-01        
Revenue from Contract with Customer [Abstract]        
Remaining performance obligation, percentage 25.00%   25.00%  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction        
Performance obligations expected to be satisfied, expected timing 24 months   24 months