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Debt - Debt Obligation for Borrowings (Details) - Line of credit - USD ($)
$ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Debt principal $ 25,000 $ 25,000
End-of-term debt obligation 283 53
Unamortized debt issuance costs (91) (115)
Net debt obligation $ 25,192 $ 24,938