CASH EQUIVALENTS AND INVESTMENTS - Cost and fair value (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Amortized Cost	$ 596,113	$ 562,089
Gross Unrealized Gains	54	50
Gross Unrealized Losses	(61)	(31)
Fair Value	596,106	562,108
U.S. treasury securities
Amortized Cost	238,252	470,108
Gross Unrealized Gains	17	30
Gross Unrealized Losses	(31)	(29)
Fair Value	238,238	470,109
U.S. government agency securities
Amortized Cost	12,233	91,981
Gross Unrealized Gains		20
Gross Unrealized Losses		(2)
Fair Value	12,233	$ 91,999
Corporate securities
Amortized Cost	43,325
Gross Unrealized Losses	(17)
Fair Value	43,308
Commercial paper
Amortized Cost	302,303
Gross Unrealized Gains	37
Gross Unrealized Losses	(13)
Fair Value	$ 302,327