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Cash, Cash Equivalents and Marketable Securities (Details) - USD ($)
$ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Cash, Cash Equivalents and Marketable Securities    
Total cash, cash equivalents and marketable securities, Unrealized gains $ 13  
Total cash, cash equivalents and marketable securities, Unrealized losses (156) $ (31)
Carrying value    
Cash, Cash Equivalents and Marketable Securities    
Cash and cash equivalents 38,046 148,627
Total cash, cash equivalents and marketable securities, Cost 488,681 191,558
Fair value    
Cash, Cash Equivalents and Marketable Securities    
Cash and cash equivalents 38,046 148,627
Total cash, cash equivalents and marketable securities, Fair value $ 488,538 $ 191,527
Corporate debt securities due within 3 months of date of purchase    
Cash, Cash Equivalents and Marketable Securities    
Maturity period from date of purchase to classify an investment as marketable securities 3 months 3 months
Corporate debt securities due within 1 year of date of purchase    
Cash, Cash Equivalents and Marketable Securities    
Maturity period from date of purchase to classify an investment as marketable securities 1 year 1 year
Corporate debt securities due within 1 year of date of purchase | Level 2    
Cash, Cash Equivalents and Marketable Securities    
Marketable securities, unrealized gains $ 13  
Marketable securities, unrealized losses (144) $ (31)
Corporate debt securities due within 1 year of date of purchase | Carrying value | Level 2    
Cash, Cash Equivalents and Marketable Securities    
Marketable securities, cost 437,106 42,931
Corporate debt securities due within 1 year of date of purchase | Fair value | Level 2    
Cash, Cash Equivalents and Marketable Securities    
Marketable securities, fair value $ 436,975 42,900
Corporate debt securities due between 1 and 2 years of the date of purchase | Minimum    
Cash, Cash Equivalents and Marketable Securities    
Maturity period from date of purchase to classify an investment as marketable securities 1 year  
Corporate debt securities due between 1 and 2 years of the date of purchase | Maximum    
Cash, Cash Equivalents and Marketable Securities    
Maturity period from date of purchase to classify an investment as marketable securities 2 years  
Corporate debt securities due between 1 and 2 years of the date of purchase | Level 2    
Cash, Cash Equivalents and Marketable Securities    
Marketable securities, unrealized losses $ (12)  
Corporate debt securities due between 1 and 2 years of the date of purchase | Carrying value | Level 2    
Cash, Cash Equivalents and Marketable Securities    
Marketable securities, cost 13,529  
Corporate debt securities due between 1 and 2 years of the date of purchase | Fair value | Level 2    
Cash, Cash Equivalents and Marketable Securities    
Marketable securities, fair value 13,517  
Cash | Carrying value | Level 1    
Cash, Cash Equivalents and Marketable Securities    
Cash and cash equivalents 2,127 2,725
Cash | Fair value | Level 1    
Cash, Cash Equivalents and Marketable Securities    
Cash and cash equivalents 2,127 2,725
Money market funds | Carrying value | Level 1    
Cash, Cash Equivalents and Marketable Securities    
Cash and cash equivalents 35,919 145,902
Money market funds | Fair value | Level 1    
Cash, Cash Equivalents and Marketable Securities    
Cash and cash equivalents $ 35,919 $ 145,902