FAIR VALUE OF CERTAIN FINANCIAL ASSETS AND LIABILITIES (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
U.S. government Agencies
Fair value of certain assets
Assets measured at fair value	$ 8,979
Variable rate demand notes
Fair value of certain assets
Assets measured at fair value	3,900
Level 1
Fair value of certain assets
Cash	162,105	196,090
Assets measured at fair value	359,992	303,018
Level 1 | Money market funds
Fair value of certain assets
Assets measured at fair value	197,887	106,928
Level 2
Fair value of certain assets
Assets measured at fair value	687,625	887,217
Level 2 | Commercial paper
Fair value of certain assets
Assets measured at fair value	15,991	19,482
Level 2 | Municipal securities
Fair value of certain assets
Assets measured at fair value	658,539	854,787
Level 2 | U.S. government Agencies
Fair value of certain assets
Assets measured at fair value	8,979	9,199
Level 2 | Variable rate demand notes
Fair value of certain assets
Assets measured at fair value	3,900	4,001
Level 2 | Foreign currency exchange contracts
Fair value of certain assets
Foreign currency derivatives	216	(252)
Level 3
Fair value of certain assets
Assets measured at fair value	4,160	4,160
Level 3 | Auction rate securities
Fair value of certain assets
Assets measured at fair value	3,910	3,910
Level 3 | Put options
Fair value of certain assets
Assets measured at fair value	250	250
Total fair value
Fair value of certain assets
Cash	162,105	196,090
Assets measured at fair value	1,051,777	1,194,395
Total fair value | Money market funds
Fair value of certain assets
Assets measured at fair value	197,887	106,928
Total fair value | Commercial paper
Fair value of certain assets
Assets measured at fair value	15,991	19,482
Total fair value | Municipal securities
Fair value of certain assets
Assets measured at fair value	658,539	854,787
Total fair value | U.S. government Agencies
Fair value of certain assets
Assets measured at fair value		9,199
Total fair value | Variable rate demand notes
Fair value of certain assets
Assets measured at fair value		4,001
Total fair value | Auction rate securities
Fair value of certain assets
Assets measured at fair value	3,910	3,910
Total fair value | Put options
Fair value of certain assets
Assets measured at fair value	250	250
Total fair value | Foreign currency exchange contracts
Fair value of certain assets
Foreign currency derivatives	$ 216	$ (252)