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Fair Value Measurements - Summary of Financial Assets Measured and Recorded at Fair Value on Recurring Basis (Details) - USD ($)
$ in Thousands
Sep. 30, 2020
Dec. 31, 2019
Aug. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale securities $ 92,022 $ 139,045  
2023 Notes | Convertible Debt      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Interest rate 1.25%   1.25%
Convertible debt $ 359,500    
2025 Notes | Convertible Debt      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Interest rate 2.25%   2.25%
Convertible debt $ 289,200    
Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets 307,471 245,826  
Total liabilities 88    
Level 1 | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets 248,436 143,760  
Total liabilities 0    
Level 2 | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets 59,035 102,066  
Total liabilities 88    
Level 3 | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets 0 0  
Total liabilities 0    
U.S. government agencies      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale securities 32,987 36,979  
U.S. government agencies | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale securities 32,987 36,979  
U.S. government agencies | Level 1 | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale securities 32,987 36,979  
U.S. government agencies | Level 2 | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale securities 0 0  
U.S. government agencies | Level 3 | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale securities 0 0  
Commercial paper      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale securities 32,252 19,966  
Commercial paper | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale securities 32,252 19,966  
Commercial paper | Level 1 | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale securities 0 0  
Commercial paper | Level 2 | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale securities 32,252 19,966  
Commercial paper | Level 3 | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale securities 0 0  
Corporate bonds      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale securities 20,349 60,878  
Corporate bonds | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale securities 20,349 60,878  
Corporate bonds | Level 1 | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale securities 0 0  
Corporate bonds | Level 2 | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale securities 20,349 60,878  
Corporate bonds | Level 3 | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale securities 0 0  
Agency bonds      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale securities 3,998 12,242  
Agency bonds | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale securities 3,998 12,242  
Agency bonds | Level 1 | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale securities 0 0  
Agency bonds | Level 2 | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale securities 3,998 12,242  
Agency bonds | Level 3 | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale securities 0 0  
Asset-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale securities 2,436 8,980  
Asset-backed securities | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale securities 2,436 8,980  
Asset-backed securities | Level 1 | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale securities 0 0  
Asset-backed securities | Level 2 | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale securities 2,436 8,980  
Asset-backed securities | Level 3 | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale securities 0 0  
Foreign currency forward contracts designated as cash flow hedges | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Foreign currency forward contracts designated as cash flow hedges 88    
Foreign currency forward contracts designated as cash flow hedges | Level 1 | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Foreign currency forward contracts designated as cash flow hedges 0    
Foreign currency forward contracts designated as cash flow hedges | Level 2 | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Foreign currency forward contracts designated as cash flow hedges 88    
Foreign currency forward contracts designated as cash flow hedges | Level 3 | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Foreign currency forward contracts designated as cash flow hedges 0    
Money market funds | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Money market funds 215,449 106,781  
Money market funds | Level 1 | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Money market funds 215,449 106,781  
Money market funds | Level 2 | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Money market funds 0 0  
Money market funds | Level 3 | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Money market funds $ 0 $ 0