Annual report pursuant to Section 13 and 15(d)

Capital Stock - ATM Agreements - Additional Information (Details)

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Capital Stock - ATM Agreements - Additional Information (Details) - USD ($)
 1 Months Ended 12 Months Ended
Feb. 28, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Class Of Warrant Or Right [Line Items]        
Proceeds from issuance of common stock   $ 10,160,000 $ 57,000 $ 171,000
Common Stock [Member]        
Class Of Warrant Or Right [Line Items]        
Sale of shares 1,200,000 1,849,000 13,000  
2015 ATM Agreement [Member]        
Class Of Warrant Or Right [Line Items]        
Sales commission paid per transaction (in hundredths)   3.00%    
Proceeds from issuance of common stock   $ 600,000 $ 56,000  
Offering costs offset against proceeds upon sale of common stock   $ 200,000 $ 56,000  
2015 ATM Agreement [Member] | Common Stock [Member]        
Class Of Warrant Or Right [Line Items]        
Sale of shares   110,252 10,365 0