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Revenue - (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Revenue from Contract with Customer [Abstract]        
Revenue from contract liability $ 170 $ 169 $ 452 $ 413
Remaining revenue recognition $ 19,500   $ 19,500  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-07-01        
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction        
Remaining performance obligation, percentage 35.00%   35.00%  
Performance obligations expected to be satisfied, expected timing     6 months  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-07-01        
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction        
Remaining performance obligation, percentage 25.00%   25.00%  
Performance obligations expected to be satisfied, expected timing     1 year