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Summary of Significant Accounting Policies - Summary (Details)
 3 Months Ended 9 Months Ended
Sep. 30, 2019
USD ($)
item
Sep. 30, 2018
USD ($)
Sep. 30, 2019
USD ($)
item
Sep. 30, 2018
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Foreign Currency Translation            
Foreign currency translation gain $ 530,000 $ 322,000 $ 364,000 $ 1,369,000    
Foreign currency transaction unrealized gains (losses) (600,000) (400,000) (469,000) (1,541,000)    
Trading Securities            
Trading securities 0   0   $ 0  
Restricted cash            
Letter of Credit outstanding 8,800,000   8,800,000   8,800,000  
Cash and cash equivalents 39,534,000   39,534,000   29,821,000  
Restricted cash 8,881,000   8,881,000   8,775,000  
cash, cash equivalents and restricted cash in the condensed consolidated statements of cash flows $ 48,415,000 37,278,000 $ 48,415,000 37,278,000 $ 38,596,000 $ 24,508,000
Number of Months from start of lease for adjustments to Letter of Credit | item 37   37      
Frequency of adjustment to Letter of Credit     12 months      
Adjustment rate of Letter of Credit (as a percent)     20.00%      
Amount of Letter of Credit outstanding after adjustments $ 2,000,000.0   $ 2,000,000.0      
Revenue Recognition            
Number of performance obligations that exist for majority of the contracts with customers | item     1      
Warranty period from date of shipment     1 year      
Numerator:            
Net loss - basic and diluted $ (13,506,000) $ (6,621,000) $ (21,157,000) $ (14,730,000)