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Cash, Cash Equivalents, Marketable Securities and Restricted Cash (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Dec. 31, 2016
Cash, Money Market Funds, and Marketable Securities        
Cash and cash equivalents $ 201,827 $ 49,060 $ 64,133 $ 187,026
Available-for-sale debt securities 362,556 309,502    
Cash balances, Fair Value 201,827 49,060    
Cost, available-for-sale securities 362,767 309,938    
Gross unrealized gain, available-for-sale securities 5 1    
Gross unrealized loss, available-for-sale securities (216) (437)    
Cost, Cash balances and available-for-sale securities 564,594 358,998    
Gross unrealized gain, Cash balances and available-for-sale securities 5 1    
Gross unrealized loss, Cash balances and available-for-sale securities (216) (437)    
Fair value, Cash balances and available-for-sale securities 564,383 358,562    
Corporate debt securities, current portion        
Cash, Money Market Funds, and Marketable Securities        
Available-for-sale debt securities 227,399 199,012    
Cost, available-for-sale securities 227,517 199,314    
Gross unrealized gain, available-for-sale securities 5 1    
Gross unrealized loss, available-for-sale securities (123) (303)    
Commercial paper        
Cash, Money Market Funds, and Marketable Securities        
Available-for-sale debt securities 82,655 79,803    
Cost, available-for-sale securities 82,704 79,878    
Gross unrealized gain, available-for-sale securities 0 0    
Gross unrealized loss, available-for-sale securities (49) (75)    
Asset-backed securities        
Cash, Money Market Funds, and Marketable Securities        
Available-for-sale debt securities 52,101 30,287    
Cost, available-for-sale securities 52,145 30,346    
Gross unrealized gain, available-for-sale securities 0 0    
Gross unrealized loss, available-for-sale securities (44) (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 51 50    
Cost, available-for-sale securities 51 50    
Gross unrealized gain, available-for-sale securities 0 0    
Gross unrealized loss, available-for-sale securities $ 0 $ 0