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Revenue - Narrative (Details)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2018
USD ($)
Jun. 30, 2018
USD ($)
Revenue from Contract with Customer [Abstract]    
Adjustments to sales incentives for previously recognized revenue $ 482 $ 628
Deferred revenue recognized 402 785
Deferred revenue expected to be recognized in the remainder of 2018 845 845
Deferred revenue expected to be recognized in 2019 885 885
Deferred revenue expected to be recognized in 2020 450 450
Deferred revenue expected to be recognized thereafter $ 569 $ 569