FAIR VALUE OF CERTAIN FINANCIAL ASSETS AND LIABILITIES (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Level 1
Fair value of certain assets
Cash	$ 153,066	$ 147,113
Assets measured at fair value	229,996	211,087
Level 1 | Money market funds
Fair value of certain assets
Assets measured at fair value	76,930	63,974
Level 2
Fair value of certain assets
Assets measured at fair value	110,937	106,706
Level 2 | U.S. Treasuries
Fair value of certain assets
Assets measured at fair value	16,023	24,065
Level 2 | Certificates of deposit
Fair value of certain assets
Assets measured at fair value	11,708	5,603
Level 2 | Municipal securities
Fair value of certain assets
Assets measured at fair value	83,206	77,038
Level 3
Fair value of certain assets
Assets measured at fair value	19,381	25,085
Level 3 | Auction rate securities
Fair value of certain assets
Assets measured at fair value	17,370	23,156
Level 3 | Put options related to auction rate securities
Fair value of certain assets
Assets measured at fair value	2,011	1,929
Total fair value
Fair value of certain assets
Cash	153,066	147,113
Assets measured at fair value	360,314	342,878
Total fair value | Money market funds
Fair value of certain assets
Assets measured at fair value	76,930	63,974
Total fair value | U.S. Treasuries
Fair value of certain assets
Assets measured at fair value	16,023	24,065
Total fair value | Certificates of deposit
Fair value of certain assets
Assets measured at fair value	11,708	5,603
Total fair value | Municipal securities
Fair value of certain assets
Assets measured at fair value	83,206	77,038
Total fair value | Auction rate securities
Fair value of certain assets
Assets measured at fair value	17,370	23,156
Total fair value | Put options related to auction rate securities
Fair value of certain assets
Assets measured at fair value	$ 2,011	$ 1,929