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Stockholders' Equity - Company's Stock Option Activity (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
2003 Stock Incentive Plan      
OUTSTANDING OPTIONS      
Outstanding at beginning of period 713,211    
Options exercised (23,725)    
Outstanding at end of period 689,486   713,211
WEIGHTED-AVERAGE EXERCISE PRICE      
Outstanding at beginning of period $ 1.25    
Options exercised 1.12    
Outstanding at end of period $ 1.25   $ 1.25
ADDITIONAL DISCLOSURES      
Weighted-average contractual term outstanding 1 year 10 months 24 days   2 years 2 months 12 days
Aggregate intrinsic value (in thousands) outstanding at end of period $ 10,328    
2013 Equity Incentive Plan      
OUTSTANDING OPTIONS      
Outstanding at beginning of period 2,664,163    
Options granted 633,979    
Options exercised (39,670) 0  
Outstanding at end of period 3,258,472   2,664,163
WEIGHTED-AVERAGE EXERCISE PRICE      
Outstanding at beginning of period $ 7.34    
Options granted 20.43    
Options exercised 6.86    
Outstanding at end of period $ 8.04   $ 7.34
VESTED AND EXPECTED TO VEST      
Outstanding options 3,258,472    
Weighted-Average Exercise Price $ 8.04    
Weighted-Average Remaining Contractual Term 7 years 7 months 6 days    
Aggregate Intrinsic Value (in thousands) $ 27,590    
ADDITIONAL DISCLOSURES      
Outstanding options exercisable at end of period 1,758,498    
Weighted-average exercise price exercisable at end of period $ 7.65    
Weighted-average contractual term (years) exercisable at end of period 6 years 7 months 6 days    
Aggregate intrinsic value (in thousands) exercisable at end of period $ 15,054    
Weighted-average contractual term outstanding 7 years 7 months 6 days   7 years 4 months 24 days
Aggregate intrinsic value (in thousands) outstanding at end of period $ 27,590