CASH, CASH EQUIVALENTS AND INVESTMENTS - Schedule of Cash, Cash Equivalents and Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Cash and Cash Equivalents [Line Items]
Investments, Amortized Cost Basis	$ 438,232	$ 433,162
Investments, Gross Unrealized Gains	22	15
Investments, Gross Unrealized Losses	(10)	(63)
Investments	438,244	433,114
Certificates of Deposit [Member]
Cash and Cash Equivalents [Line Items]
Cash Equivalents, at Carrying Value	1,100	500
Cash Equivalents, Gross Unrealized Gain	1
Cash Equivalents, Gross Unrealized Losses	(1)	(1)
Cash and Cash Equivalents, Fair Value Disclosure	1,100	499
Corporate Debt Securities [Member]
Cash and Cash Equivalents [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		114,678
Available-for-sale Securities, Gross Unrealized Gains		1
Available-for-sale Securities, Gross Unrealized Losses		(19)
Available-for-sale Securities, Fair Value Disclosure		114,660
U.S. Government Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash Equivalents, at Carrying Value	136,700	128,800
Available-for-sale Debt Securities, Amortized Cost Basis	437,132	317,984
Available-for-sale Securities, Gross Unrealized Gains	21	14
Available-for-sale Securities, Gross Unrealized Losses	(9)	(43)
Available-for-sale Securities, Fair Value Disclosure	$ 437,144	$ 317,955

Cash Equivalents, Gross Unrealized Gain

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Cash Equivalents, Gross Unrealized Losses

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Investments, Amortized Cost Basis

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Investments, Gross Unrealized Gains

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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Sum of the carrying amounts as of the balance sheet date of all investments.

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