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Short-Term Investments (Details)
6 Months Ended
Jun. 30, 2017
USD ($)
item
Dec. 31, 2016
USD ($)
Short-term investments    
Short-term Investments. $ 2,895,000 $ 3,246,000
Certificates of Deposit    
Short-term investments    
Number of fully insured certificates of deposit | item 12  
Fully insured certificates of deposit $ 2,900,000  
Certificates of Deposit | Minimum    
Short-term investments    
Maturity period 6 months  
Interest rate (as a percent) 0.65%  
Certificates of Deposit | Maximum    
Short-term investments    
Maturity period 12 months  
Interest rate (as a percent) 1.25%