CASH, CASH EQUIVALENTS AND INVESTMENTS - Schedule of Cash, Cash Equivalents and Investments (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Line Items]
Investments, Amortized Cost Basis	$ 433,162,000	$ 320,563,000
Investments, Gross Unrealized Gains	15,000	1,000
Investments, Gross Unrealized Losses	(63,000)	(392,000)
Investments	433,114,000	320,172,000
Certificates of Deposit [Member]
Cash and Cash Equivalents [Line Items]
Cash Equivalents, at Carrying Value	500,000	1,296,000
Cash Equivalents, Gross Unrealized Gain		1,000
Cash Equivalents, Gross Unrealized Losses	(1,000)	(1,000)
Cash and Cash Equivalents, Fair Value Disclosure	499,000	1,296,000
Corporate Debt Securities [Member]
Cash and Cash Equivalents [Line Items]
Cash Equivalents, at Carrying Value	0	17,900,000
Available-for-sale Debt Securities, Amortized Cost Basis	114,678,000	104,626,000
Available-for-sale Securities, Gross Unrealized Gains	1,000
Available-for-sale Securities, Gross Unrealized Losses	(19,000)	(252,000)
Available-for-sale Securities, Fair Value Disclosure	114,660,000	104,374,000
U.S. Government Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash Equivalents, at Carrying Value	128,800,000	0
Available-for-sale Debt Securities, Amortized Cost Basis	317,984,000	214,641,000
Available-for-sale Securities, Gross Unrealized Gains	14,000
Available-for-sale Securities, Gross Unrealized Losses	(43,000)	(139,000)
Available-for-sale Securities, Fair Value Disclosure	$ 317,955,000	$ 214,502,000