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Cash, Cash Equivalents, Marketable Securities, and Restricted Cash - Cash and Available for Sale Securities (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]      
Cash and cash equivalents $ 163,240 $ 142,837 $ 79,749
Cost, debt securities 320,016 309,665  
Cost, cash balances and debt securities 483,256 452,502  
Gross Unrealized Gain 39 255  
Gross Unrealized Loss (26) (17)  
Fair value, cash balances 163,240 142,837  
Debt securities, available-for-sale 320,029 309,903  
Fair value, cash balances and debt securities 483,269 452,740  
Marketable securities with maturity date greater than 12 months   9,500  
Corporate debt securities      
Debt Securities, Available-for-sale [Line Items]      
Cost, debt securities 39,525 145,875  
Gross Unrealized Gain 4 121  
Gross Unrealized Loss (16) (5)  
Debt securities, available-for-sale 39,513 145,991  
Commercial paper      
Debt Securities, Available-for-sale [Line Items]      
Cost, debt securities 217,087 73,659  
Gross Unrealized Gain 14 53  
Gross Unrealized Loss (6) (2)  
Debt securities, available-for-sale 217,095 73,710  
Asset-backed securities      
Debt Securities, Available-for-sale [Line Items]      
Cost, debt securities 9,420 77,731  
Gross Unrealized Gain 18 79  
Gross Unrealized Loss 0 0  
Debt securities, available-for-sale 9,438 77,810  
U.S. government agency bonds      
Debt Securities, Available-for-sale [Line Items]      
Cost, debt securities 53,583 11,999  
Gross Unrealized Gain 3 2  
Gross Unrealized Loss (4) (10)  
Debt securities, available-for-sale 53,582 11,991  
Money market      
Debt Securities, Available-for-sale [Line Items]      
Cost, debt securities 350 350  
Gross Unrealized Gain 0 0  
Gross Unrealized Loss 0 0  
Debt securities, available-for-sale 350 350  
Certificates of deposit      
Debt Securities, Available-for-sale [Line Items]      
Cost, debt securities 51 51  
Gross Unrealized Gain 0 0  
Gross Unrealized Loss 0 0  
Debt securities, available-for-sale $ 51 $ 51