INVESTMENTS - Maturity Period (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Investments
Amortized Cost	$ 1,060,060	$ 925,559
Fair Value	1,059,985	925,645
Commercial paper | Less than 1 year
Investments
Amortized Cost	186,912	119,886
Fair Value	186,912	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	64,914	81,521
Fair Value	64,922	81,531
U.S. government agency securities | Due 1 - 10 years
Investments
Amortized Cost	29,659	10,350
Fair Value	29,651	10,351
Certificates of deposit | Less than 1 year
Investments
Amortized Cost	43,240	20,387
Fair Value	43,242	20,387
U.S. treasuries | Less than 1 year
Investments
Amortized Cost	673,912	650,386
Fair Value	673,876	650,467
U.S. treasuries | Due 1 - 10 years
Investments
Amortized Cost	61,423	33,946
Fair Value	$ 61,382	$ 33,940