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Fair Value Measurement - Financial Instruments (Details) - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term investments $ 1,215,944 $ 665,567
Long-term investments 711,224 523,628
Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured and recorded at fair value 2,558,937 1,653,994
Fair Value, Measurements, Recurring | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured and recorded at fair value 532,493 464,799
Fair Value, Measurements, Recurring | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured and recorded at fair value 2,026,444 1,189,195
Commercial paper | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term investments 605,364 204,170
Commercial paper | Fair Value, Measurements, Recurring | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term investments 0 0
Commercial paper | Fair Value, Measurements, Recurring | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term investments 605,364 204,170
Corporate debt securities | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term investments 610,580 461,397
Long-term investments 672,226 484,597
Corporate debt securities | Fair Value, Measurements, Recurring | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term investments 0 0
Long-term investments 0 0
Corporate debt securities | Fair Value, Measurements, Recurring | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term investments 610,580 461,397
Long-term investments 672,226 484,597
Agency bonds | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term investments 38,998 39,031
Agency bonds | Fair Value, Measurements, Recurring | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term investments 0 0
Agency bonds | Fair Value, Measurements, Recurring | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term investments 38,998 39,031
Money market funds | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 532,493 464,799
Money market funds | Fair Value, Measurements, Recurring | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 532,493 464,799
Money market funds | Fair Value, Measurements, Recurring | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 0 $ 0
Commercial paper | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 99,276  
Commercial paper | Fair Value, Measurements, Recurring | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 0  
Commercial paper | Fair Value, Measurements, Recurring | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents $ 99,276