FAIR VALUE MEASUREMENTS: (Details) - USD ($)	Jun. 30, 2016	Feb. 23, 2016	Dec. 31, 2015
Financial assets measured at fair value
Principal Due on Term Debt	$ 273,000,000
Debt Instrument, Fair Value Disclosure	271,600,000
Fair value, assets, level 1 to level 2 transfers, amount	0		$ 0
Debt instrument, face amount	273,000,000	$ 280,000,000
Fair value, measurements, recurring basis [Member]
Financial assets measured at fair value
Financial assets	218,569,000		444,989,000
Investments, fair value disclosure	217,939,000
Fair value, measurements, recurring basis [Member] | Money market funds [Member]
Financial assets measured at fair value
Financial assets	4,985,000
Fair value, measurements, recurring basis [Member] | Certificates of deposits [Member]
Financial assets measured at fair value
Financial assets	57,486,000		110,423,000
Fair value, measurements, recurring basis [Member] | U.S. Government and agency securities [Member]
Financial assets measured at fair value
Financial assets	43,908,000		131,722,000
Fair value, measurements, recurring basis [Member] | Commercial paper [Member]
Financial assets measured at fair value
Financial assets	28,363,000		57,214,000
Fair value, measurements, recurring basis [Member] | Corporate bonds [Member]
Financial assets measured at fair value
Financial assets	74,638,000		105,482,000
Fair value, measurements, recurring basis [Member] | Municipal bonds [Member]
Financial assets measured at fair value
Financial assets	3,441,000		26,208,000
Fair value, measurements, recurring basis [Member] | Foreign government bonds [Member]
Financial assets measured at fair value
Financial assets	5,118,000		13,940,000
Fair value, measurements, recurring basis [Member] | Level 1 [Member]
Financial assets measured at fair value
Financial assets	4,985,000		0
Investments, fair value disclosure	4,985,000
Fair value, measurements, recurring basis [Member] | Level 1 [Member] | Money market funds [Member]
Financial assets measured at fair value
Financial assets	4,985,000
Fair value, measurements, recurring basis [Member] | Level 2 [Member]
Financial assets measured at fair value
Financial assets	213,584,000		444,989,000
Investments, fair value disclosure	212,954,000
Fair value, measurements, recurring basis [Member] | Level 2 [Member] | Certificates of deposits [Member]
Financial assets measured at fair value
Financial assets	57,486,000		110,423,000
Fair value, measurements, recurring basis [Member] | Level 2 [Member] | U.S. Government and agency securities [Member]
Financial assets measured at fair value
Financial assets	43,908,000		131,722,000
Fair value, measurements, recurring basis [Member] | Level 2 [Member] | Commercial paper [Member]
Financial assets measured at fair value
Financial assets	28,363,000		57,214,000
Fair value, measurements, recurring basis [Member] | Level 2 [Member] | Corporate bonds [Member]
Financial assets measured at fair value
Financial assets	74,638,000		105,482,000
Fair value, measurements, recurring basis [Member] | Level 2 [Member] | Municipal bonds [Member]
Financial assets measured at fair value
Financial assets	3,441,000		26,208,000
Fair value, measurements, recurring basis [Member] | Level 2 [Member] | Foreign government bonds [Member]
Financial assets measured at fair value
Financial assets	5,118,000		13,940,000
Derivative financial instruments, assets [Member] | Fair value, measurements, recurring basis [Member]
Financial assets measured at fair value
Financial assets	630,000
Derivative financial instruments, assets [Member] | Fair value, measurements, recurring basis [Member] | Level 2 [Member]
Financial assets measured at fair value
Financial assets	630,000
Derivative financial instruments, liabilities [Member] | Fair value, measurements, recurring basis [Member]
Financial assets measured at fair value
Financial and nonfinancial liabilities, fair value disclosure	524,000		1,157,000
Derivative financial instruments, liabilities [Member] | Fair value, measurements, recurring basis [Member] | Level 1 [Member]
Financial assets measured at fair value
Financial and nonfinancial liabilities, fair value disclosure			0
Derivative financial instruments, liabilities [Member] | Fair value, measurements, recurring basis [Member] | Level 2 [Member]
Financial assets measured at fair value
Financial and nonfinancial liabilities, fair value disclosure	$ 524,000		$ 1,157,000