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Stock-Based Compensation - Additional Information (Detail) - USD ($)
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2015
Jan. 01, 2018
Jan. 01, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Fair value of options granted $ 10.49 $ 13.53 $ 10.51 $ 12.41      
Excess tax benefits from stock option activity or stock-based compensation expense recognized in cash flows from operations $ 0 $ 0          
ASU 2016-09 [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Cumulative-effect adjustment to stockholders' equity             $ 45,000
Non-employee Consultants [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Weighted average fair values of outstanding options $ 9.97 $ 12.18 $ 9.97 $ 12.18      
2014 Equity Incentive Plan [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Number of stock options that may be granted after the tenth anniversary of the 2014 Plan     0        
Annual increases in number of shares reserved for issuance as a percentage of shares of capital stock outstanding through January 1, 2024         3.00%    
Share-based compensation arrangement by share-based payment award, number of shares authorized           4,900,481 3,920,613
Options granted 136,500 90,000 732,500 838,500      
2014 Equity Incentive Plan [Member] | Share-Based Compensation Award, Tranche One [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Percentage of vested shares on first anniversary of grant date     25.00%        
2014 Equity Incentive Plan [Member] | Share-based Compensation Award, Tranche Two [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Vesting period of awards granted     36 months        
2014 Equity Incentive Plan [Member] | Maximum [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Term of awards granted     10 years        
2014 Equity Incentive Plan [Member] | Maximum [Member] | Incentive Stock Options [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Share-based compensation arrangement by share-based payment award, number of shares authorized         30,000,000    
2014 Equity Incentive Plan [Member] | Employees [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Vesting period of awards granted     4 years