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INVESTMENTS (Details) - USD ($)
$ in Thousands
Sep. 30, 2020
Dec. 31, 2019
Available-for-sale    
Amortized Cost $ 619,507 $ 545,771
Gross Unrealized Holding Gains 419 285
Gross Unrealized Holding Losses 29 88
Fair Value 619,897 545,968
Continuous Unrealized Loss Position less than 12 Months 29 88
Short-term | Commercial paper    
Available-for-sale    
Amortized Cost 91,445 83,478
Fair Value 91,445 83,478
Short-term | Certificates of deposit    
Available-for-sale    
Amortized Cost 19,959 28,049
Fair Value 19,959 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 66,435 40,620
Gross Unrealized Holding Gains 21 5
Gross Unrealized Holding Losses 3 35
Fair Value 66,453 40,590
Continuous Unrealized Loss Position less than 12 Months 3 35
Short-term | U.S. treasuries    
Available-for-sale    
Amortized Cost 421,093 211,055
Gross Unrealized Holding Gains 398 134
Gross Unrealized Holding Losses 22 31
Fair Value 421,469 211,158
Continuous Unrealized Loss Position less than 12 Months 22 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 10,205 5,267
Gross Unrealized Holding Losses 1 1
Fair Value 10,204 5,266
Continuous Unrealized Loss Position less than 12 Months 1 1
Long-term | U.S. treasuries    
Available-for-sale    
Amortized Cost 10,370 6,077
Gross Unrealized Holding Gains   1
Gross Unrealized Holding Losses 3  
Fair Value 10,367 $ 6,078
Continuous Unrealized Loss Position less than 12 Months $ 3