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Cash Equivalents and Investments (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]    
Cost $ 171,784 $ 236,744
Gross unrealized gain 5 0
Gross unrealized loss (13) 0
Fair value 171,776 236,744
Short-term investments 102,556 0
Money market funds    
Debt Securities, Available-for-sale [Line Items]    
Cost 38,354 236,744
Gross unrealized gain 0 0
Gross unrealized loss 0 0
Fair value 38,354 236,744
US treasury securities    
Debt Securities, Available-for-sale [Line Items]    
Cost 80,844 0
Gross unrealized gain 5 0
Gross unrealized loss (5) 0
Fair value 80,844 0
U.S. government agency obligations    
Debt Securities, Available-for-sale [Line Items]    
Cost 52,586 0
Gross unrealized gain 0 0
Gross unrealized loss (8) 0
Fair value 52,578 0
Cash equivalents    
Debt Securities, Available-for-sale [Line Items]    
Fair value $ 69,220 $ 236,744