CASH, CASH EQUIVALENTS AND INVESTMENTS - Schedule of Cash, Cash Equivalents (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Cash And Cash Equivalents [Line Items]
Cash Equivalents, at Carrying Value	$ 535,160	$ 630,010
Cash Equivalents, Gross Unrealized Gains		6
Cash Equivalents, Gross Unrealized Losses	3	4
Cash and Cash Equivalents, Fair Value Disclosure	535,157	630,012
Cash Accounts in Banking Institutions [Member]
Cash And Cash Equivalents [Line Items]
Cash Equivalents, at Carrying Value	222,833	163,779
Cash and Cash Equivalents, Fair Value Disclosure	222,833	163,779
Money Market Accounts [Member]
Cash And Cash Equivalents [Line Items]
Cash Equivalents, at Carrying Value	17,339	17,261
Cash and Cash Equivalents, Fair Value Disclosure	17,339	17,261
U.S. Government Bonds [Member]
Cash And Cash Equivalents [Line Items]
Cash Equivalents, at Carrying Value	294,988	448,970
Cash Equivalents, Gross Unrealized Gains		6
Cash Equivalents, Gross Unrealized Losses	3	4
Cash and Cash Equivalents, Fair Value Disclosure	$ 294,985	$ 448,972