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INVESTMENTS (Details) - USD ($)
$ in Thousands
Jun. 30, 2020
Dec. 31, 2019
Available-for-sale    
Amortized Cost $ 252,173 $ 545,771
Gross Unrealized Holding Gains 674 285
Gross Unrealized Holding Losses 17 88
Fair Value 252,830 545,968
Continuous Unrealized Loss Position less than 12 Months 17 88
Short-term | Commercial paper    
Available-for-sale    
Amortized Cost 58,540 83,478
Fair Value 58,540 83,478
Short-term | Certificates of deposit    
Available-for-sale    
Amortized Cost 3,624 28,049
Fair Value 3,624 28,049
Short-term | Municipal securities    
Available-for-sale    
Amortized Cost   147,983
Gross Unrealized Holding Gains   145
Gross Unrealized Holding Losses   20
Fair Value   148,108
Continuous Unrealized Loss Position less than 12 Months   20
Short-term | U.S. government agency securities    
Available-for-sale    
Amortized Cost 44,851 40,620
Gross Unrealized Holding Gains 41 5
Gross Unrealized Holding Losses 9 35
Fair Value 44,883 40,590
Continuous Unrealized Loss Position less than 12 Months 9 35
Short-term | U.S. treasuries    
Available-for-sale    
Amortized Cost 143,081 211,055
Gross Unrealized Holding Gains 633 134
Gross Unrealized Holding Losses 8 31
Fair Value 143,706 211,158
Continuous Unrealized Loss Position less than 12 Months 8 31
Short-term | Variable rate demand notes    
Available-for-sale    
Amortized Cost   21,680
Fair Value   21,680
Long-term | Municipal securities    
Available-for-sale    
Amortized Cost   1,562
Gross Unrealized Holding Losses   1
Fair Value   1,561
Continuous Unrealized Loss Position less than 12 Months   1
Long-term | U.S. government agency securities    
Available-for-sale    
Amortized Cost   5,267
Gross Unrealized Holding Losses   1
Fair Value   5,266
Continuous Unrealized Loss Position less than 12 Months   1
Long-term | U.S. treasuries    
Available-for-sale    
Amortized Cost 2,077 6,077
Gross Unrealized Holding Gains   1
Fair Value $ 2,077 $ 6,078