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Commitments and Contingencies - Maturity Schedule for Contractual Commitments (Detail) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Contractual Obligation, Fiscal Year Maturity [Abstract]    
2018 $ 1,683  
2019 0  
2020 0  
2021 0  
Total purchase commitments $ 1,683 $ 1,700