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Investments (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Cash and cash equivalents:        
Cash and cash equivalents $ 202,818 $ 197,569 $ 197,945 $ 91,060
Cash and Cash Equivalents, Fair Value Disclosure 202,818 197,569    
Investments:        
Amortized cost   52,114    
Gross unrealized gains   0    
Gross unrealized losses   (28)    
Fair value   52,086    
Total cash, cash equivalents and investments, amortized cost   249,683    
Total cash, cash equivalents and investments, fair value   249,655    
Cash and money market funds        
Cash and cash equivalents:        
Cash and cash equivalents 202,818 197,569    
Cash and Cash Equivalents, Fair Value Disclosure $ 202,818 197,569    
Commercial paper        
Investments:        
Amortized cost   30,883    
Gross unrealized gains   0    
Gross unrealized losses   0    
Fair value   30,883    
Corporate bonds        
Investments:        
Amortized cost   21,231    
Gross unrealized gains   0    
Gross unrealized losses   (28)    
Fair value   $ 21,203