INVESTMENTS - Maturity Period (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Investments
Amortized Cost	$ 1,788,306	$ 1,850,101
Fair Value	1,783,300	1,849,146
Commercial paper | Less than 1 year
Investments
Amortized Cost	276,413	334,077
Fair Value	276,413	334,077
Municipal securities | Less than 1 year
Investments
Amortized Cost	168,958	666
Fair Value	168,478	666
Municipal securities | Due 1 - 10 years
Investments
Amortized Cost	5,284
Fair Value	5,266
U.S. government agency securities | Less than 1 year
Investments
Amortized Cost	78,831	62,687
Fair Value	78,489	62,661
U.S. government agency securities | Due 1 - 10 years
Investments
Amortized Cost	21,455	12,500
Fair Value	21,297	12,476
Certificates of deposit | Less than 1 year
Investments
Amortized Cost	37,010	44,502
Fair Value	37,010	44,502
U.S. treasuries | Less than 1 year
Investments
Amortized Cost	1,161,042	1,308,536
Fair Value	1,157,258	1,307,821
U.S. treasuries | Due 1 - 10 years
Investments
Amortized Cost	39,313	87,133
Fair Value	$ 39,089	$ 86,943