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Summary of Significant Accounting Policies (Details)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2018
USD ($)
Segment
Mar. 31, 2017
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Restricted Cash [Abstract]        
Cash and cash equivalents $ 171,136   $ 18,142  
Restricted cash 208   207  
Total 171,344 $ 27,360 $ 18,349 $ 9,665
Short-term Investments [Abstract]        
Realized gains (losses) on short-term investment 0 0    
Net unrealized gains on short-term investment 10 0    
Schedule of Available-for-sale Securities [Line Items]        
Cost basis 11,551      
Unrealized gains 10      
Fair value 11,561      
Other than temporary impairment 0      
NYC Biotechnology Tax Credit Program [Abstract]        
Research and development incentive $ 186      
Research and development incentive receivable   $ 192    
Income Taxes [Abstract]        
Federal corporate tax rate 21.00%   34.00%  
Segment Reporting [Abstract]        
Number of operating segments | Segment 1      
Maximum [Member]        
Property, Plant and Equipment [Line Items]        
Estimated useful lives 5 years      
Minimum [Member]        
Property, Plant and Equipment [Line Items]        
Estimated useful lives 3 years      
Certificates of Deposit [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cost basis $ 1,546      
Unrealized gains 0      
Fair value 1,546      
United States Treasury Securities [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cost basis 10,005      
Unrealized gains 10      
Fair value $ 10,015