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Revenue - (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Revenue from Contract with Customer [Abstract]        
Revenue from contract liability $ 188 $ 170 $ 508 $ 452
Remaining revenue recognition $ 20,000   $ 20,000  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-04-01        
Revenue from Contract with Customer [Abstract]        
Remaining performance obligation, percentage 35.00%   35.00%  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-07-01        
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction        
Performance obligations expected to be satisfied, expected timing 1 year   1 year  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-04-01        
Revenue from Contract with Customer [Abstract]        
Remaining performance obligation, percentage 25.00%   25.00%  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-07-01        
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction        
Performance obligations expected to be satisfied, expected timing 1 year   1 year