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Accrued Expenses - Schedule of Accrued Expenses (Details) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Dec. 31, 2017
Payables And Accruals [Abstract]      
Compensation and related benefits $ 622 $ 1,734  
Accrued warranty (Note 7) 456 112 $ 87
Accrued services 3,308 963  
Accrued expenses $ 4,386 $ 2,809