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INVESTMENTS - Maturity Period (Details) - USD ($)
$ in Thousands
Mar. 31, 2020
Dec. 31, 2019
Investments    
Amortized Cost $ 246,937 $ 545,771
Fair Value 247,435 545,968
Commercial paper | Less than 1 year    
Investments    
Amortized Cost 57,107 83,478
Fair Value 57,107 83,478
Municipal securities | Less than 1 year    
Investments    
Amortized Cost 95,819 147,983
Fair Value 95,820 148,108
Municipal securities | Due 1 - 10 years    
Investments    
Amortized Cost 6,292 1,562
Fair Value 6,264 1,561
U.S. government agency securities | Less than 1 year    
Investments    
Amortized Cost 21,441 40,620
Fair Value 21,503 40,590
U.S. government agency securities | Due 1 - 10 years    
Investments    
Amortized Cost 3,512 5,267
Fair Value 3,522 5,266
Certificates of deposit | Less than 1 year    
Investments    
Amortized Cost   28,049
Fair Value 0 28,049
U.S. treasuries | Less than 1 year    
Investments    
Amortized Cost 46,055 211,055
Fair Value 46,454 211,158
U.S. treasuries | Due 1 - 10 years    
Investments    
Amortized Cost 4,083 6,077
Fair Value 4,137 6,078
Variable rate demand notes | Due 1 - 10 years    
Investments    
Amortized Cost 3,911 3,905
Fair Value 3,911 3,905
Variable rate demand notes | Due 11 - 20 years    
Investments    
Amortized Cost 3,485 8,886
Fair Value 3,485 8,886
Variable rate demand notes | Due 21 - 30 years    
Investments    
Amortized Cost 3,217 6,885
Fair Value 3,217 6,885
Variable rate demand notes | Due 31 - 40 years    
Investments    
Amortized Cost 2,015 2,004
Fair Value $ 2,015 $ 2,004