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Fair Value of Financial Instruments - Additional Information (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Mar. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Unrealized gain (loss) on investments $ 6,600    
Long-term investments 132,188 $ 0  
Level 2      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Long-term investments $ 0 $ 0  
Convertible Senior Notes Due 2023      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Debt instrument, interest rate     0.00%
Convertible Senior Notes Due 2023 | Level 2      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Debt instrument, interest rate 0.00%    
Estimated fair value of convertible senior notes $ 929,200