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Stock Plans - Plan Information (Details) - shares
12 Months Ended
Dec. 31, 2017
Dec. 15, 2017
Stockholder's Equity    
Threshold consecutive trading days 20 days  
2015 Plan    
Stockholder's Equity    
Number of shares available for grant (in shares) 2,000,000  
2017 New Hire Plan    
Stockholder's Equity    
Number of shares available for grant (in shares) 1,500,000  
Maximum shares authorized   1,500,000.0