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Debt Instruments - Loan and Security Agreement (Details) - USD ($)
12 Months Ended
Nov. 04, 2019
Dec. 31, 2018
Dec. 31, 2019
Nov. 30, 2017
Aug. 31, 2016
Nov. 30, 2014
Sep. 30, 2013
Jul. 31, 2013
Loan and Security Agreement                
Debt Instrument [Line Items]                
Debt facility, maximum borrowing amount       $ 30,000,000.0 $ 17,500,000 $ 15,000,000.0 $ 5,000,000.0 $ 2,500,000
Amount of debt outstanding   $ 29,200,000            
Senior Loan Agreement                
Debt Instrument [Line Items]                
Amount of debt outstanding   $ 49,167,000 $ 0          
Extinguishment of debt $ 20,000,000.0              
Line of Credit | Loan and Security Agreement | Prime Rate                
Debt Instrument [Line Items]                
Basis spread on variable rate   1.75%