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Basis of Presentation and Summary of Significant Accounting Policies - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2018
Dec. 31, 2017
Accounting Policies [Abstract]      
Revenue, remaining obligation, expected timing of satisfaction   three to six months,  
Contract liability $ 181 $ 181 $ 181
Contract liabilities, current portion 154 154 $ 150
Revenue recognized, previously recorded as deferred revenue $ 26 $ 128