FAIR VALUE MEASUREMENTS - Narrative (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Feb. 23, 2016	Dec. 31, 2015
Financial assets measured at fair value
Term Debt, principal amount	$ 74,000,000
Debt instrument, face amount			$ 280,000,000.0
Fair value of long-term debt	74,900,000
Fair value, measurements, recurring basis
Financial assets measured at fair value
Investments at fair value	213,138,000	$ 273,494,000
Long-term restricted cash	8,025,000
Financial assets	222,145,000	273,751,000
Financial liabilities	17,000	1,006,000
Fair value, measurements, recurring basis | Money market funds
Financial assets measured at fair value
Investments at fair value	1,857,000			$ 1,833,000
Fair value, measurements, recurring basis | Certificates of deposit
Financial assets measured at fair value
Investments at fair value	58,003,000	78,643,000
Fair value, measurements, recurring basis | U.S. Government and agency securities
Financial assets measured at fair value
Investments at fair value	43,872,000	56,347,000
Fair value, measurements, recurring basis | Commercial paper
Financial assets measured at fair value
Investments at fair value	27,029,000	29,483,000
Fair value, measurements, recurring basis | Corporate bonds
Financial assets measured at fair value
Investments at fair value	54,447,000	94,162,000
Fair value, measurements, recurring basis | Municipal bonds
Financial assets measured at fair value
Investments at fair value	15,169,000	7,706,000
Fair value, measurements, recurring basis | Foreign government bonds
Financial assets measured at fair value
Investments at fair value	12,761,000	5,320,000
Fair value, measurements, recurring basis | Derivative contracts
Financial assets measured at fair value
Financial assets	982,000	257,000
Financial liabilities	17,000	1,006,000
Fair value, measurements, recurring basis | Level 1
Financial assets measured at fair value
Investments at fair value	1,857,000	1,833,000
Long-term restricted cash	0
Financial assets	1,857,000	1,833,000
Financial liabilities	0	0
Fair value, measurements, recurring basis | Level 1 | Money market funds
Financial assets measured at fair value
Investments at fair value	1,857,000			1,833,000
Fair value, measurements, recurring basis | Level 1 | Certificates of deposit
Financial assets measured at fair value
Investments at fair value	0	0
Fair value, measurements, recurring basis | Level 1 | U.S. Government and agency securities
Financial assets measured at fair value
Investments at fair value	0	0
Fair value, measurements, recurring basis | Level 1 | Commercial paper
Financial assets measured at fair value
Investments at fair value	0	0
Fair value, measurements, recurring basis | Level 1 | Corporate bonds
Financial assets measured at fair value
Investments at fair value	0	0
Fair value, measurements, recurring basis | Level 1 | Municipal bonds
Financial assets measured at fair value
Investments at fair value	0	0
Fair value, measurements, recurring basis | Level 1 | Foreign government bonds
Financial assets measured at fair value
Investments at fair value	0	0
Fair value, measurements, recurring basis | Level 1 | Derivative contracts
Financial assets measured at fair value
Financial assets	0	0
Financial liabilities	0	0
Fair value, measurements, recurring basis | Level 2
Financial assets measured at fair value
Investments at fair value	211,281,000	271,661,000
Long-term restricted cash	8,025,000
Financial assets	220,288,000	271,918,000
Financial liabilities	17,000	1,006,000
Fair value, measurements, recurring basis | Level 2 | Money market funds
Financial assets measured at fair value
Investments at fair value	0			$ 0
Fair value, measurements, recurring basis | Level 2 | Certificates of deposit
Financial assets measured at fair value
Investments at fair value	58,003,000	78,643,000
Fair value, measurements, recurring basis | Level 2 | U.S. Government and agency securities
Financial assets measured at fair value
Investments at fair value	43,872,000	56,347,000
Fair value, measurements, recurring basis | Level 2 | Commercial paper
Financial assets measured at fair value
Investments at fair value	27,029,000	29,483,000
Fair value, measurements, recurring basis | Level 2 | Corporate bonds
Financial assets measured at fair value
Investments at fair value	54,447,000	94,162,000
Fair value, measurements, recurring basis | Level 2 | Municipal bonds
Financial assets measured at fair value
Investments at fair value	15,169,000	7,706,000
Fair value, measurements, recurring basis | Level 2 | Foreign government bonds
Financial assets measured at fair value
Investments at fair value	12,761,000	5,320,000
Fair value, measurements, recurring basis | Level 2 | Derivative contracts
Financial assets measured at fair value
Financial assets	982,000	257,000
Financial liabilities	$ 17,000	$ 1,006,000