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Cash Equivalents & Marketable Securities (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
item
Dec. 31, 2016
USD ($)
Cash equivalents and marketable securities    
Amortized Cost $ 329,215 $ 185,323
Gross Unrealized Losses (76)  
Fair Value $ 329,139 185,323
Number of securities in an unrealized loss position | item 25  
Securities held by the Company in an unrealized loss position for less than 12 months $ 174,000  
Number of securities in an unrealized loss position for more than 12 months | item 0  
Realized gains (losses) on available-for-sale securities $ 0 0
U.S. Treasuries    
Cash equivalents and marketable securities    
Amortized Cost 123,606  
Gross Unrealized Losses (47)  
Fair Value 123,559  
Government agency securities    
Cash equivalents and marketable securities    
Amortized Cost 58,979  
Gross Unrealized Losses (29)  
Fair Value 58,950  
Money market funds    
Cash equivalents and marketable securities    
Amortized Cost 134,635 185,323
Fair Value 134,635 $ 185,323
U.S. Treasuries    
Cash equivalents and marketable securities    
Amortized Cost 11,995  
Fair Value $ 11,995