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Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)
9 Months Ended
Sep. 30, 2019
Aug. 02, 2019
Dec. 31, 2018
May 02, 2017
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]        
Long-term debt, net $ 40,070,000   $ 3,640,000  
Silicon Valley Bank [Member] | 2019 Term Loan [Member]        
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]        
Term loan, principal amount   $ 40,000,000.0    
Long-term debt, net   $ 40,000,000.0    
Silicon Valley Bank And MidCap Financial Funding XIII Trust [Member]        
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]        
Repayments of Lines of Credit $ 7,700,000      
2024 Convertible Notes [Member]        
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]        
Term loan, principal amount       $ 201,300,000
Fair value of liability       136,700,000
Unamortized debt discount       $ 64,500,000
Debt instrument interest rate 3.375%     3.375%
Estimated fair value of Convertible Notes $ 187,900,000