Quarterly report pursuant to Section 13 or 15(d)

Stock-Based Compensation Plans - Employee Stock Purchase Plan - Additional Information (Detail)

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Stock-Based Compensation Plans - Employee Stock Purchase Plan - Additional Information (Detail) - USD ($)
 6 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Common stock shares available for issuance 5,472,696  
Unrecognized stock-based compensation expense $ 22,249,000  
Unrecognized stock-based compensation expenses recognition period 3 years 15 days  
Employee Stock Purchase Plan [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
ESPP offering period description The offering periods generally start on the first trading day on or after May 15 and November 15 of each year.  
Length of purchase intervals 6 months  
ESPP discount rate 10.00%  
Common stock reserved and available for issuance 366,667  
Common stock shares available for issuance 233,867 246,895
Unrecognized stock-based compensation expense $ 55,000  
Unrecognized stock-based compensation expenses recognition period 4 months 17 days  
Employee Stock Purchase Plan - Annual Available for Issuance Share Increase [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Common stock available for future issuance authorized annual share increase 366,667  
Common stock available for future issuance authorized annual percentage increase 1.00%  
ESPP description The ESPP Plan provides for annual increases in the number of shares available for issuance under the ESPP, equal to the least of (i) 366,667 shares, (ii) 1% of the outstanding shares on such date, or (iii) an amount determined by the board. Pursuant to the provisions of the ESPP Plan, the board of directors determined a share reserve increase was not required in the prior years.