FAIR VALUE OF CERTAIN FINANCIAL ASSETS AND LIABILITIES (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Level 1
Fair value of certain assets
Cash	$ 89,680	$ 147,113
Assets measured at fair value	262,866	211,087
Level 1 | Money market funds
Fair value of certain assets
Assets measured at fair value	173,186	63,974
Level 2
Fair value of certain assets
Assets measured at fair value	163,361	106,706
Level 2 | U.S. Treasuries
Fair value of certain assets
Assets measured at fair value	11,019	24,065
Level 2 | Certificates of deposit
Fair value of certain assets
Assets measured at fair value	11,515	5,603
Level 2 | Municipal securities
Fair value of certain assets
Assets measured at fair value	140,827	77,038
Level 3
Fair value of certain assets
Assets measured at fair value	19,448	25,085
Level 3 | Auction rate securities
Fair value of certain assets
Assets measured at fair value	17,762	23,156
Level 3 | Put option related to auction rate securities
Fair value of certain assets
Assets measured at fair value	1,686	1,929
Total fair value
Fair value of certain assets
Cash	89,680	147,113
Assets measured at fair value	445,675	342,878
Total fair value | Money market funds
Fair value of certain assets
Assets measured at fair value	173,186	63,974
Total fair value | U.S. Treasuries
Fair value of certain assets
Assets measured at fair value	11,019	24,065
Total fair value | Certificates of deposit
Fair value of certain assets
Assets measured at fair value	11,515	5,603
Total fair value | Municipal securities
Fair value of certain assets
Assets measured at fair value	140,827	77,038
Total fair value | Auction rate securities
Fair value of certain assets
Assets measured at fair value	17,762	23,156
Total fair value | Put option related to auction rate securities
Fair value of certain assets
Assets measured at fair value	$ 1,686	$ 1,929