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Debt - Other Long-Term Debt (Details) - Line of Credit - Silicon Valley Bank - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Aug. 31, 2014
Debt Instrument [Line Items]        
Line of credit, maximum borrowing capacity       $ 15,000,000.0
Line of credit facility interest rate at period end 4.50%      
Interest expense $ 0 $ 0 $ 0  
Line of credit outstanding 0 $ 0    
Minimum        
Debt Instrument [Line Items]        
Line of credit asset restrictions on cash $ 5,000,000.0