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Stockholders' Equity - ATM Facility & Share Offering - Additional Information (Detail) - USD ($)
 1 Months Ended 12 Months Ended
Feb. 28, 2017
Jan. 31, 2016
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2018
Jun. 22, 2018
Jun. 21, 2018
Jun. 27, 2017
Class of Stock [Line Items]                
Legal, accounting and other costs $ 223,000              
Common Stock, Shares Authorized     150,000,000 120,000,000 150,000,000 150,000,000 120,000,000 60,000,000
Share Offering [Member]                
Class of Stock [Line Items]                
Common stock issued to investors   4,440,000            
Gross proceeds from issuance of common stock   $ 17,800,000            
Share issuance costs   $ 1,300,000            
Common stock price per share   $ 4.00            
At-the-Market Offering [Member]                
Class of Stock [Line Items]                
Common stock shares maximum aggregate offering price $ 20,000,000              
Stock issuances, sales agent commission maximum percentage 3.00%              
Common stock issued to investors     5,900,000 5,100,000        
Weighted average common stock price per share     $ 1.23 $ 1.74        
Gross proceeds from issuance of common stock     $ 7,300,000 $ 8,900,000        
Share issuance costs     $ 239,000 $ 244,000