FAIR VALUE MEASUREMENTS (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Financial assets measured at fair value
Cash and cash equivalents, estimated fair value	$ 77,579	$ 56,766
Debt securities, available-for-sale	798,318	381,724
Derivative contracts	1,056	96
Derivative contracts	0	2,536
Fair value, measurements, recurring basis
Financial assets measured at fair value
Derivative contracts	1,056	96
Investments at fair value		382,989
Long-term restricted cash		7,884
Total financial assets	800,432	390,969
Derivative contracts		2,536
Total financial liabilities		2,536
Fair value, measurements, recurring basis \| Level 1
Financial assets measured at fair value
Derivative contracts	0	0
Investments at fair value		1,265
Long-term restricted cash		0
Total financial assets	1,058	1,265
Derivative contracts		0
Total financial liabilities		0
Fair value, measurements, recurring basis \| Level 2
Financial assets measured at fair value
Derivative contracts	1,056	96
Investments at fair value		381,724
Long-term restricted cash		7,884
Total financial assets	799,374	389,704
Derivative contracts		2,536
Total financial liabilities		2,536
Money market funds
Financial assets measured at fair value
Cash and cash equivalents, estimated fair value	1,058	1,265
Money market funds \| Fair value, measurements, recurring basis
Financial assets measured at fair value
Cash and cash equivalents, estimated fair value	1,058	1,265
Money market funds \| Fair value, measurements, recurring basis \| Level 1
Financial assets measured at fair value
Cash and cash equivalents, estimated fair value	1,058	1,265
Money market funds \| Fair value, measurements, recurring basis \| Level 2
Financial assets measured at fair value
Cash and cash equivalents, estimated fair value	0	0
Certificates of deposit
Financial assets measured at fair value
Debt securities, available-for-sale	198,663	95,038
Certificates of deposit \| Fair value, measurements, recurring basis
Financial assets measured at fair value
Cash and cash equivalents, estimated fair value	198,663	95,038
Certificates of deposit \| Fair value, measurements, recurring basis \| Level 1
Financial assets measured at fair value
Cash and cash equivalents, estimated fair value	0	0
Certificates of deposit \| Fair value, measurements, recurring basis \| Level 2
Financial assets measured at fair value
Cash and cash equivalents, estimated fair value	198,663	95,038
Government debt securities
Financial assets measured at fair value
Debt securities, available-for-sale	232,604	100,478
Government debt securities \| Fair value, measurements, recurring basis
Financial assets measured at fair value
Debt securities, available-for-sale	232,604	100,478
Government debt securities \| Fair value, measurements, recurring basis \| Level 1
Financial assets measured at fair value
Debt securities, available-for-sale	0	0
Government debt securities \| Fair value, measurements, recurring basis \| Level 2
Financial assets measured at fair value
Debt securities, available-for-sale	232,604	100,478
Corporate debt securities
Financial assets measured at fair value
Debt securities, available-for-sale	367,051	186,208
Corporate debt securities \| Fair value, measurements, recurring basis
Financial assets measured at fair value
Debt securities, available-for-sale	367,051	186,208
Corporate debt securities \| Fair value, measurements, recurring basis \| Level 1
Financial assets measured at fair value
Debt securities, available-for-sale	0	0
Corporate debt securities \| Fair value, measurements, recurring basis \| Level 2
Financial assets measured at fair value
Debt securities, available-for-sale	$ 367,051	$ 186,208

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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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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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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Fair value of financial and nonfinancial obligations.

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