INVESTMENTS - Maturity Period (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Investments
Amortized Cost	$ 1,252,346	$ 925,559
Fair Value	1,252,321	925,645
Commercial paper | Less than 1 year
Investments
Amortized Cost	292,094	119,886
Fair Value	292,094	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	70,202	81,521
Fair Value	70,210	81,531
U.S. government agency securities | Due 1 - 10 years
Investments
Amortized Cost	1,416	10,350
Fair Value	1,416	10,351
Certificates of deposit | Less than 1 year
Investments
Amortized Cost	47,616	20,387
Fair Value	47,616	20,387
U.S. treasuries | Less than 1 year
Investments
Amortized Cost	814,178	650,386
Fair Value	814,146	650,467
U.S. treasuries | Due 1 - 10 years
Investments
Amortized Cost	26,840	33,946
Fair Value	$ 26,839	$ 33,940