INVESTMENTS - Maturity Period (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Investments
Amortized Cost	$ 619,507	$ 545,771
Fair Value	619,897	545,968
Commercial paper | Less than 1 year
Investments
Amortized Cost	91,445	83,478
Fair Value	91,445	83,478
Municipal securities | Less than 1 year
Investments
Amortized Cost		147,983
Fair Value		148,108
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	66,435	40,620
Fair Value	66,453	40,590
U.S. government agency securities | Due 1 - 10 years
Investments
Amortized Cost	10,205	5,267
Fair Value	10,204	5,266
Certificates of deposit | Less than 1 year
Investments
Amortized Cost	19,959	28,049
Fair Value	19,959	28,049
U.S. treasuries | Less than 1 year
Investments
Amortized Cost	421,093	211,055
Fair Value	421,469	211,158
U.S. treasuries | Due 1 - 10 years
Investments
Amortized Cost	10,370	6,077
Fair Value	$ 10,367	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
Fair Value		6,885
Variable rate demand notes | Due 31 - 40 years
Investments
Amortized Cost		2,004
Fair Value		$ 2,004