Annual report pursuant to Section 13 and 15(d)

Equity Incentive Plans and Stock Based Compensation - Additional Information (Detail)

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Equity Incentive Plans and Stock Based Compensation - Additional Information (Detail) - USD ($)
1 Months Ended 12 Months Ended
Jan. 01, 2017
Jan. 01, 2016
Aug. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Total intrinsic value of stock options exercised       $ 78,000 $ 0
Cash received from stock option exercises       61,000 2,000
Tax benefit realized upon exercise of option       0  
Stock-based compensation expense       1,792,000 $ 109,000
Unrecognized compensation cost related to unvested stock-based awards granted to employees under the stock option plans       $ 7,100,000  
Unrecognized compensation cost related to unvested stock-based awards granted to employees under the stock option plans, period for recognition       3 years 6 months  
2009 Equity Incentive Plan          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Number of shares of common stock issuable under plan       1,452,268  
Terms of stock options vested       5 years  
Percentage of shares vesting on one-year anniversary and equal monthly vesting installments thereafter       20.00%  
Options to purchase shares of common stock available for future grants       101,667  
2014 Equity Incentive Plan          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Terms of stock options vested       4 years  
Options to purchase shares of common stock available for future grants       637,029  
Percentage of fair market value     100.00%    
Terms of stock options granted       10 years  
Stock-based compensation expense       $ 1,200,000  
2004 Equity Incentive Plan          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Terms of stock options vested       4 years  
Percentage of fair market value       100.00%  
Terms of stock options granted       10 years  
2004 Employee Stock Purchase Plan          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Options to purchase shares of common stock available for future grants       18,917  
Additional shares authorized for issuance 9,091 9,091      
Offering period       24 months  
Purchase period       6 months  
Discount available to eligible employees related to employee stock purchase plan       85.00%  
Plan participants purchased shares       2,868 8,368
Average purchase price under 2004 Employee Stock Purchase Plan       $ 2.80 $ 2.75