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Cash, Cash Equivalents, Marketable Securities and Restricted Cash (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Dec. 31, 2016
Cash, Money Market Funds, and Marketable Securities        
Cash and cash equivalents $ 73,311 $ 49,060 $ 37,394 $ 187,026
Available-for-sale debt securities 479,477 309,502    
Cash balances, Fair Value 73,311 49,060    
Cost, available-for-sale securities 479,933 309,938    
Gross unrealized gain, available-for-sale securities 15 1    
Gross unrealized loss, available-for-sale securities (471) (437)    
Cost, Cash balances and available-for-sale securities 553,244 358,998    
Gross unrealized gain, Cash balances and available-for-sale securities 15 1    
Gross unrealized loss, Cash balances and available-for-sale securities (471) (437)    
Fair value, Cash balances and available-for-sale securities 552,788 358,562    
Corporate debt securities, current portion        
Cash, Money Market Funds, and Marketable Securities        
Available-for-sale debt securities 271,529 199,012    
Cost, available-for-sale securities 271,855 199,314    
Gross unrealized gain, available-for-sale securities 0 1    
Gross unrealized loss, available-for-sale securities (326) (303)    
Corporate debt securities, non-current portion        
Cash, Money Market Funds, and Marketable Securities        
Available-for-sale debt securities 13,837      
Cost, available-for-sale securities 13,875      
Gross unrealized gain, available-for-sale securities 0      
Gross unrealized loss, available-for-sale securities (38)      
Commercial paper        
Cash, Money Market Funds, and Marketable Securities        
Available-for-sale debt securities 144,698 79,803    
Cost, available-for-sale securities 144,726 79,878    
Gross unrealized gain, available-for-sale securities 15 0    
Gross unrealized loss, available-for-sale securities (43) (75)    
Asset-backed securities        
Cash, Money Market Funds, and Marketable Securities        
Available-for-sale debt securities 49,013 30,287    
Cost, available-for-sale securities 49,077 30,346    
Gross unrealized gain, available-for-sale securities 0 0    
Gross unrealized loss, available-for-sale securities (64) (59)    
Money market        
Cash, Money Market Funds, and Marketable Securities        
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        
Available-for-sale debt securities 50 50    
Cost, available-for-sale securities 50 50    
Gross unrealized gain, available-for-sale securities 0 0    
Gross unrealized loss, available-for-sale securities $ 0 $ 0