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INVESTMENTS - Maturity Period (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Investments    
Amortized Cost $ 545,771 $ 320,740
Fair Value 545,968 320,650
Commercial paper | Less than 1 year    
Investments    
Amortized Cost 83,478 52,838
Fair Value 83,478 52,838
Municipal securities | Less than 1 year    
Investments    
Amortized Cost 147,983 151,690
Fair Value 148,108 151,644
Municipal securities | Due 1 - 10 years    
Investments    
Amortized Cost 1,562  
Fair Value 1,561  
U.S. government agency securities | Less than 1 year    
Investments    
Amortized Cost 40,620 19,943
Fair Value 40,590 19,931
U.S. government agency securities | Due 1 - 10 years    
Investments    
Amortized Cost 5,267  
Fair Value 5,266  
Certificates of deposit | Less than 1 year    
Investments    
Amortized Cost 28,049 14,075
Fair Value 28,049 14,075
U.S. treasuries | Less than 1 year    
Investments    
Amortized Cost 211,055 78,189
Fair Value 211,158 78,157
U.S. treasuries | Due 1 - 10 years    
Investments    
Amortized Cost 6,077  
Fair Value 6,078  
Variable rate demand notes | Due 1 - 10 years    
Investments    
Amortized Cost 3,905  
Fair Value 3,905  
Variable rate demand notes | Due 11 - 20 years    
Investments    
Amortized Cost 8,886  
Fair Value 8,886  
Variable rate demand notes | Due 21 - 30 years    
Investments    
Amortized Cost 6,885 4,005
Fair Value 6,885 $ 4,005
Variable rate demand notes | Due 31 - 40 years    
Investments    
Amortized Cost 2,004  
Fair Value $ 2,004