FAIR VALUE MEASUREMENTS: (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Financial assets measured at fair value
Transfers between Level 1 and Level 2	$ 0	$ 0
Fair Value, Measurements, Recurring Basis | Level 1
Financial assets measured at fair value
Financial assets	869	4,426
Fair Value, Measurements, Recurring Basis | Level 1 | Money market funds
Financial assets measured at fair value
Financial assets	869	4,426
Fair Value, Measurements, Recurring Basis | Level 2
Financial assets measured at fair value
Financial assets	369,468	334,038
Financial liabilities	3,272	3,562
Fair Value, Measurements, Recurring Basis | Level 2 | Certificates of deposits
Financial assets measured at fair value
Financial assets	91,377	80,275
Fair Value, Measurements, Recurring Basis | Level 2 | U.S. Government and agency securities
Financial assets measured at fair value
Financial assets	102,531	99,114
Fair Value, Measurements, Recurring Basis | Level 2 | Commercial paper
Financial assets measured at fair value
Financial assets	17,965	23,019
Fair Value, Measurements, Recurring Basis | Level 2 | Corporate bonds
Financial assets measured at fair value
Financial assets	120,369	111,736
Fair Value, Measurements, Recurring Basis | Level 2 | Municipal bonds
Financial assets measured at fair value
Financial assets	25,161	13,104
Fair Value, Measurements, Recurring Basis | Level 2 | Derivative contracts
Financial assets measured at fair value
Financial assets	3,272	3,562
Fair Value, Measurements, Recurring Basis | Level 2 | Foreign government bonds
Financial assets measured at fair value
Financial assets	12,065	6,790
Fair Value, Measurements, Recurring Basis | Total
Financial assets measured at fair value
Financial assets	370,337	338,464
Financial liabilities	3,272	3,562
Fair Value, Measurements, Recurring Basis | Total | Money market funds
Financial assets measured at fair value
Financial assets	869	4,426
Fair Value, Measurements, Recurring Basis | Total | Certificates of deposits
Financial assets measured at fair value
Financial assets	91,377	80,275
Fair Value, Measurements, Recurring Basis | Total | U.S. Government and agency securities
Financial assets measured at fair value
Financial assets	102,531	99,114
Fair Value, Measurements, Recurring Basis | Total | Commercial paper
Financial assets measured at fair value
Financial assets	17,965	23,019
Fair Value, Measurements, Recurring Basis | Total | Corporate bonds
Financial assets measured at fair value
Financial assets	120,369	111,736
Fair Value, Measurements, Recurring Basis | Total | Municipal bonds
Financial assets measured at fair value
Financial assets	25,161	13,104
Fair Value, Measurements, Recurring Basis | Total | Derivative contracts
Financial assets measured at fair value
Financial assets	3,272	3,562
Fair Value, Measurements, Recurring Basis | Total | Foreign government bonds
Financial assets measured at fair value
Financial assets	$ 12,065	$ 6,790