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Stock-Based Compensation - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Dec. 31, 2017
Share-Based Compensation        
Fair value of vested stock options $ 1,000 $ 800    
Stock-based compensation expense 1,920 1,357    
T2-Owned Instruments and Components [Member]        
Share-Based Compensation        
Stock-based compensation expense $ 100 $ 0    
Restricted Stock Units [Member]        
Share-Based Compensation        
Weighted-average period 1 year 10 months 24 days      
Unrecognized compensation cost related to unvested stock options $ 3,000      
2006 Stock Option Plan [Member] | Stock Options [Member]        
Share-Based Compensation        
Shares available for grant 1,250,943      
2014 Stock Option Plan [Member] | Stock Options [Member]        
Share-Based Compensation        
Shares available for future issuance under stock incentive plan 823,529      
Percentage of common shares outstanding 4.00%      
Shares available for grant 1,250,943      
2014 Stock Option Plan [Member] | Restricted Stock Units [Member]        
Share-Based Compensation        
Restricted stock units vested 85,629      
Restricted stock units granted 589,142      
Inducement Award Plan [Member]        
Share-Based Compensation        
Shares available for grant 1,009,750      
Shares available for authorization   1,625,000    
2006 and 2014 Stock Option Plans [Member] | Stock Options [Member]        
Share-Based Compensation        
Aggregate fair value of options granted $ 2,000 $ 4,400    
Unrecognized compensation cost related to non-vested stock options $ 7,100      
Weighted-average period 3 years      
2006 and 2014 Stock Option Plans and Inducement Plan [Member] | Stock Options [Member]        
Share-Based Compensation        
Weighted average fair value of options granted $ 2.20 $ 3.41    
2006 and 2014 Stock Option Plans and Inducement Plan [Member] | Restricted Stock Units [Member]        
Share-Based Compensation        
Aggregate fair value of restricted stock units granted $ 1,900      
Restricted stock units vested     73,172 78,172
2006 and 2014 Stock Option Plans and Inducement Plan [Member] | Restricted Stock Units with Market Condition [Member]        
Share-Based Compensation        
Restricted stock units granted 898,633      
Compensation Cost $ 700 $ 200    
Maximum [Member] | 2006 Stock Option Plan [Member] | Stock Options [Member]        
Share-Based Compensation        
Expiration period 10 years      
Vesting period 4 years      
Maximum [Member] | 2014 Stock Option Plan [Member] | Stock Options [Member]        
Share-Based Compensation        
Expiration period 10 years      
Vesting period 4 years