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Credit Facility (Details) - USD ($)
Mar. 31, 2018
Dec. 31, 2017
Aug. 07, 2017
Debt Instrument [Line Items]      
Line of credit $ 30,000,000 $ 30,000,000  
Silicon Valley Bank | Letter of Credit      
Debt Instrument [Line Items]      
Maximum borrowing capacity     $ 30,000,000.0