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Cash Equivalents and Investments - Available for Sale securities (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, amortized cost $ 150,017 $ 61,009
Available-for-sale securities, unrealized gains 0 0
Available-for-sale securities, unrealized losses (101) (51)
Available-for-sale securities, fair value 149,916 60,958
Corporate notes    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, amortized cost 31,749 46,133
Available-for-sale securities, unrealized gains 0 0
Available-for-sale securities, unrealized losses (55) (35)
Available-for-sale securities, fair value 31,694 46,098
Government notes    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, amortized cost 12,448 6,191
Available-for-sale securities, unrealized gains 0 0
Available-for-sale securities, unrealized losses (18) (16)
Available-for-sale securities, fair value 12,430 6,175
Asset backed securities    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, amortized cost 12,226  
Available-for-sale securities, unrealized gains 0  
Available-for-sale securities, unrealized losses (28)  
Available-for-sale securities, fair value 12,198  
Money market funds    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, amortized cost 9,850 8,685
Available-for-sale securities, unrealized gains 0 0
Available-for-sale securities, unrealized losses 0 0
Available-for-sale securities, fair value 9,850 $ 8,685
Commercial paper    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, amortized cost 83,744  
Available-for-sale securities, unrealized gains 0  
Available-for-sale securities, unrealized losses 0  
Available-for-sale securities, fair value $ 83,744