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Stockholders' Equity - Summary of RSU Activity (Detail) - 2013 Equity Incentive Plan
12 Months Ended
Dec. 31, 2017
USD ($)
item
$ / shares
shares
NUMBER OF SHARES  
Unvested at beginning of period | shares 16,916
Vested | shares 7,249
Unvested at end of period | shares 9,667
WEIGHTED-AVERAGE GRANT DATE FAIR VALUE  
Unvested at beginning of period | $ / shares $ 5.04
Vested | $ / shares 5.61
Unvested at end of period | $ / shares $ 4.61
RSU [Member]  
Restricted Stock Units information  
Unrecognized compensation expense related to unvested service RSUs | $ $ 23,380
Number of vesting installments | item 3