CASH, CASH EQUIVALENTS AND INVESTMENTS - Schedule of Cash, Cash Equivalents (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Cash And Cash Equivalents [Line Items]
Cash Equivalents, at Carrying Value	$ 235,514	$ 211,013
Cash Equivalents, Gross Unrealized Gains	27	9
Cash and Cash Equivalents, Fair Value Disclosure	235,541	211,022
Cash Accounts in Banking Institutions [Member]
Cash And Cash Equivalents [Line Items]
Cash Equivalents, at Carrying Value	72,536	71,217
Cash and Cash Equivalents, Fair Value Disclosure	72,536	71,217
Money Market Accounts [Member]
Cash And Cash Equivalents [Line Items]
Cash Equivalents, at Carrying Value	11,230	11,014
Cash and Cash Equivalents, Fair Value Disclosure	11,230	11,014
U.S. Government Bonds [Member]
Cash And Cash Equivalents [Line Items]
Cash Equivalents, at Carrying Value	151,748	128,782
Cash Equivalents, Gross Unrealized Gains	27	9
Cash and Cash Equivalents, Fair Value Disclosure	$ 151,775	$ 128,791