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Fair Value Measurements - Schedule of Financial Instruments and the Related Fair Value Hierarchy of the Valuation Techniques Utilized (Detail) - Fair Value, Nonrecurring [Member] - USD ($)
$ in Thousands
Sep. 30, 2020
Dec. 31, 2019
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total Financial Assets $ 311,979 $ 281,314
Total Financial Liabilities 370,994 149,496
Money Market Funds [Member]    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total Financial Assets 271,158 281,314
Bank Certificates of Deposit [Member]    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total Financial Assets 40,821  
Convertible Senior Notes [Member]    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total Financial Liabilities 370,994 149,496
Level 1 [Member]    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total Financial Assets 311,979 281,314
Level 1 [Member] | Money Market Funds [Member]    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total Financial Assets 271,158 281,314
Level 1 [Member] | Bank Certificates of Deposit [Member]    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total Financial Assets 40,821  
Level 3 [Member]    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total Financial Liabilities 370,994 149,496
Level 3 [Member] | Convertible Senior Notes [Member]    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Total Financial Liabilities $ 370,994 $ 149,496