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Stock-Based Compensation - Restricted Stock and Restricted Stock Units - Additional Information (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Stock-Based Compensation      
Stock-based compensation expense $ 21,317 $ 5,056 $ 374
Restricted Stock and Restricted Stock Units      
Stock-Based Compensation      
Stock-based compensation expense 2,700 $ 100  
Unrecognized employee stock-based compensation $ 15,600    
Weighted-average recognition period employee stock based compensation 3 years 4 months 24 days