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Cash, Cash Equivalents, Marketable Securities and Restricted Cash - Components (Details) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Jun. 30, 2018
Dec. 31, 2017
Cash, Money Market Funds, and Marketable Securities        
Cash and cash equivalents $ 220,578 $ 79,749 $ 73,311 $ 49,060
Fair value, Available-for-sale debt securities 355,078 424,403    
Fair Value, Cash balances 220,578 79,749    
Cost, available-for-sale securities 354,723 424,830    
Gross unrealized gain, available-for-sale securities 372 11    
Gross unrealized loss, available-for-sale securities (17) (438)    
Cost, Cash balances and available-for-sale securities 575,301 504,579    
Gross unrealized gain, Cash balances and available-for-sale securities 372 11    
Gross unrealized loss, Cash balances and available-for-sale securities (17) (438)    
Fair value, Cash balances and available-for-sale securities 575,656 504,152    
Corporate debt securities        
Cash, Money Market Funds, and Marketable Securities        
Fair value, Available-for-sale debt securities 169,558 240,726    
Cost, available-for-sale securities 169,425 240,969    
Gross unrealized gain, available-for-sale securities 148 7    
Gross unrealized loss, available-for-sale securities (15) (250)    
Commercial paper        
Cash, Money Market Funds, and Marketable Securities        
Fair value, Available-for-sale debt securities 112,026 115,141    
Cost, available-for-sale securities 111,872 115,245    
Gross unrealized gain, available-for-sale securities 156 0    
Gross unrealized loss, available-for-sale securities (2) (104)    
Asset-backed securities        
Cash, Money Market Funds, and Marketable Securities        
Fair value, Available-for-sale debt securities 73,093 68,135    
Cost, available-for-sale securities 73,025 68,215    
Gross unrealized gain, available-for-sale securities 68 4    
Gross unrealized loss, available-for-sale securities 0 (84)    
Money market        
Cash, Money Market Funds, and Marketable Securities        
Fair value, Available-for-sale debt securities 350 350    
Cost, available-for-sale securities 350 350    
Gross unrealized gain, available-for-sale securities 0 0    
Gross unrealized loss, available-for-sale securities 0 0    
Certificates of deposit        
Cash, Money Market Funds, and Marketable Securities        
Fair value, Available-for-sale debt securities 51 51    
Cost, available-for-sale securities 51 51    
Gross unrealized gain, available-for-sale securities 0 0    
Gross unrealized loss, available-for-sale securities $ 0 $ 0