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Investments - Amortized Cost and Fair Value on Available-for-Sale Investments and Unrealized Gains and Losses (Details) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Amounts Maturing in 90 days or Less | Cash And Cash Equivalents    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale debt securities, Amortized Cost $ 6,990  
Available-for-sale debt securities, Fair Value 6,990  
Amounts Maturing in 90 days or Less | Corporate Notes and Obligations | Cash And Cash Equivalents    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale debt securities, Amortized Cost 2,498  
Available-for-sale debt securities, Fair Value 2,498  
Amounts Maturing in 90 days or Less | U.S. Agency Securities | Cash And Cash Equivalents    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale debt securities, Amortized Cost 4,492  
Available-for-sale debt securities, Fair Value 4,492  
Amounts Maturing in One Year or Less    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale debt securities, Amortized Cost   $ 36,766
Available-for-sale debt securities, Gross Unrealized Losses   (25)
Available-for-sale debt securities, Fair Value   36,741
Amounts Maturing in One Year or Less | Investments    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale debt securities, Amortized Cost 84,422  
Available-for-sale debt securities, Gross Unrealized Losses (147)  
Available-for-sale debt securities, Fair Value 84,275  
Amounts Maturing in One Year or Less | Corporate Notes and Obligations    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale debt securities, Amortized Cost   29,866
Available-for-sale debt securities, Gross Unrealized Losses   (25)
Available-for-sale debt securities, Fair Value   29,841
Amounts Maturing in One Year or Less | Corporate Notes and Obligations | Investments    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale debt securities, Amortized Cost 35,229  
Available-for-sale debt securities, Gross Unrealized Losses (23)  
Available-for-sale debt securities, Fair Value 35,206  
Amounts Maturing in One Year or Less | U.S. Government Treasury Securities    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale debt securities, Amortized Cost   6,900
Available-for-sale debt securities, Fair Value   6,900
Amounts Maturing in One Year or Less | U.S. Government Treasury Securities | Investments    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale debt securities, Amortized Cost 49,193  
Available-for-sale debt securities, Gross Unrealized Losses (124)  
Available-for-sale debt securities, Fair Value $ 49,069  
Amounts Maturing in Greater than One Year    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale debt securities, Amortized Cost   38,515
Available-for-sale debt securities, Gross Unrealized Losses   (69)
Available-for-sale debt securities, Fair Value   38,446
Amounts Maturing in Greater than One Year | Corporate Notes and Obligations    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale debt securities, Amortized Cost   4,010
Available-for-sale debt securities, Gross Unrealized Losses   (11)
Available-for-sale debt securities, Fair Value   3,999
Amounts Maturing in Greater than One Year | U.S. Government Treasury Securities    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale debt securities, Amortized Cost   34,505
Available-for-sale debt securities, Gross Unrealized Losses   (58)
Available-for-sale debt securities, Fair Value   $ 34,447