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Stock-Based Compensation - Schedule of Stock-Based Compensation Expense by Operating Expense Category (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Stock-based compensation expense, by operating expense category      
Stock-based compensation expense $ 21,317 $ 5,056 $ 374
Platform operations      
Stock-based compensation expense, by operating expense category      
Stock-based compensation expense 2,674 756 71
Sales and marketing      
Stock-based compensation expense, by operating expense category      
Stock-based compensation expense 6,261 1,707 127
Technology and development      
Stock-based compensation expense, by operating expense category      
Stock-based compensation expense 6,661 1,513 85
General and administrative      
Stock-based compensation expense, by operating expense category      
Stock-based compensation expense $ 5,721 $ 1,080 $ 91