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Cash, Cash Equivalents, Marketable Securities and Restricted Cash (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Mar. 31, 2018
Dec. 31, 2017
Cash, Money Market Funds, and Marketable Securities        
Cash and cash equivalents $ 96,349 $ 79,749 $ 114,322 $ 49,060
Fair value, Available-for-sale debt securities 341,978 424,403    
Fair Value, Cash balances 96,349 79,749    
Cost, available-for-sale securities 341,821 424,830    
Gross unrealized gain, available-for-sale securities 178 11    
Gross unrealized loss, available-for-sale securities (21) (438)    
Cost, Cash balances and available-for-sale securities 438,170 504,579    
Gross unrealized gain, Cash balances and available-for-sale securities 178 11    
Gross unrealized loss, Cash balances and available-for-sale securities (21) (438)    
Fair value, Cash balances and available-for-sale securities 438,327 504,152    
Corporate debt securities, current portion        
Cash, Money Market Funds, and Marketable Securities        
Fair value, Available-for-sale debt securities 138,284 240,726    
Cost, available-for-sale securities 138,194 240,969    
Gross unrealized gain, available-for-sale securities 96 7    
Gross unrealized loss, available-for-sale securities (6) (250)    
Commercial paper        
Cash, Money Market Funds, and Marketable Securities        
Fair value, Available-for-sale debt securities 134,993 115,141    
Cost, available-for-sale securities 134,948 115,245    
Gross unrealized gain, available-for-sale securities 52 0    
Gross unrealized loss, available-for-sale securities (7) (104)    
Asset-backed securities        
Cash, Money Market Funds, and Marketable Securities        
Fair value, Available-for-sale debt securities 68,300 68,135    
Cost, available-for-sale securities 68,278 68,215    
Gross unrealized gain, available-for-sale securities 30 4    
Gross unrealized loss, available-for-sale securities (8) (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