INVESTMENTS - Maturity Period (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Investments
Amortized Cost	$ 1,043,750	$ 925,559
Fair Value	1,043,859	925,645
Commercial paper | Less than 1 year
Investments
Amortized Cost	166,306	119,886
Fair Value	166,306	119,886
Municipal securities | Less than 1 year
Investments
Amortized Cost		9,083
Fair Value		9,083
U.S. government agency securities | Less than 1 year
Investments
Amortized Cost	87,645	81,521
Fair Value	87,658	81,531
U.S. government agency securities | Due 1 - 10 years
Investments
Amortized Cost	8,141	10,350
Fair Value	8,136	10,351
Certificates of deposit | Less than 1 year
Investments
Amortized Cost	40,517	20,387
Fair Value	40,520	20,387
U.S. treasuries | Less than 1 year
Investments
Amortized Cost	685,521	650,386
Fair Value	685,624	650,467
U.S. treasuries | Due 1 - 10 years
Investments
Amortized Cost	55,620	33,946
Fair Value	$ 55,615	$ 33,940