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FAIR VALUE MEASUREMENTS - SECURITIES WITH UNREALIZED LOSSES (Details) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]    
Less Than 12 Months - Estimated Fair Value $ 20,749 $ 107,767
Less Than 12 Months - Gross Unrealized Losses (23) (176)
12 Months or Longer - Estimated Fair Value   14,221
12 Months or Longer - Gross Unrealized Losses   (36)
Total - Estimated Fair Value 20,749 121,988
Total - Gross Unrealized Losses (23) (212)
Government-sponsored enterprise securities (due in one to two years)    
Schedule Of Available For Sale Securities [Line Items]    
Less Than 12 Months - Estimated Fair Value 8,484  
Less Than 12 Months - Gross Unrealized Losses (15)  
Total - Estimated Fair Value 8,484  
Total - Gross Unrealized Losses (15)  
Commercial paper (due in less than one year)    
Schedule Of Available For Sale Securities [Line Items]    
Less Than 12 Months - Estimated Fair Value   22,628
Less Than 12 Months - Gross Unrealized Losses   (29)
Total - Estimated Fair Value   22,628
Total - Gross Unrealized Losses   (29)
Corporate notes (due in less than one year)    
Schedule Of Available For Sale Securities [Line Items]    
Less Than 12 Months - Estimated Fair Value 9,256 66,557
Less Than 12 Months - Gross Unrealized Losses (5) (82)
12 Months or Longer - Estimated Fair Value   14,221
12 Months or Longer - Gross Unrealized Losses   (36)
Total - Estimated Fair Value 9,256 80,778
Total - Gross Unrealized Losses (5) (118)
Corporate notes (due in one to two years)    
Schedule Of Available For Sale Securities [Line Items]    
Less Than 12 Months - Estimated Fair Value 3,009 18,582
Less Than 12 Months - Gross Unrealized Losses (3) (65)
Total - Estimated Fair Value 3,009 18,582
Total - Gross Unrealized Losses $ (3) $ (65)