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Goodwill and Acquired Intangible Assets - Aggregate Amortization Expense for Acquired Intangible Assets (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]      
Amortization expense - Actual 2018 $ 7 $ 8 $ 10
Amortization expense - 2019 5    
Amortization expense - 2020 4    
Amortization expense - 2021 4    
Amortization expense - 2022 $ 2