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Deferred Revenue - Narrative (Details) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Dec. 31, 2017
Revenue from Contract with Customer [Abstract]        
Total deferred revenue $ 332 $ 339 $ 217 $ 216
Deferred revenue, current 247   $ 135  
Noncurrent deferred revenue 85      
Revenue recognized $ 94 $ 98