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Acquired Intangible Assets - Aggregate Amortization Expense for Acquired Intangible Assets (Detail) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]          
Amortization expense $ 2 $ 1 $ 4 $ 3 $ 8
Amortization expense - Remainder of 2018 7   7    
Amortization expense - 2019 6   6    
Amortization expense - 2020 4   4    
Amortization expense - 2021 4   4    
Amortization expense - 2022 $ 2   $ 2