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Cash, Cash Equivalents and Marketable Securities (Details) - USD ($)
$ in Thousands
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Cash, Cash Equivalents and Marketable Securities    
Total cash, cash equivalents and marketable securities, Unrealized gains $ 41  
Total cash, cash equivalents and marketable securities, Unrealized losses (84) $ (31)
Carrying value    
Cash, Cash Equivalents and Marketable Securities    
Cash and cash equivalents 264,861 148,627
Total cash, cash equivalents and marketable securities, Cost 490,348 191,558
Fair value    
Cash, Cash Equivalents and Marketable Securities    
Cash and cash equivalents 264,861 148,627
Total cash, cash equivalents and marketable securities, Fair value $ 490,305 $ 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 3 months of date of purchase | Carrying value | Level 2    
Cash, Cash Equivalents and Marketable Securities    
Marketable securities, cost $ 32,424  
Corporate debt securities due within 3 months of date of purchase | Fair value | Level 2    
Cash, Cash Equivalents and Marketable Securities    
Marketable securities, fair value $ 32,424  
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 $ 41  
Marketable securities, unrealized losses (84) $ (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 225,487 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 225,444 42,900
Cash | Carrying value | Level 1    
Cash, Cash Equivalents and Marketable Securities    
Cash and cash equivalents 1,615 2,725
Cash | Fair value | Level 1    
Cash, Cash Equivalents and Marketable Securities    
Cash and cash equivalents 1,615 2,725
Money market funds | Carrying value | Level 1    
Cash, Cash Equivalents and Marketable Securities    
Cash and cash equivalents 230,822 145,902
Money market funds | Fair value | Level 1    
Cash, Cash Equivalents and Marketable Securities    
Cash and cash equivalents $ 230,822 $ 145,902