FAIR VALUE OF CERTAIN FINANCIAL ASSETS AND LIABILITIES (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Level 1
Fair value of certain assets
Cash	$ 52,088	$ 50,202
Assets measured at fair value	439,012	377,724
Level 1 | Money market funds
Fair value of certain assets
Assets measured at fair value	321,065	242,001
Level 1 | U.S. Treasuries
Fair value of certain assets
Assets measured at fair value	65,859	85,521
Level 2
Fair value of certain assets
Assets measured at fair value	234,969	197,704
Level 2 | Certificates of deposit
Fair value of certain assets
Assets measured at fair value	50,559	40,010
Level 2 | Corporate bonds
Fair value of certain assets
Assets measured at fair value	2,042
Level 2 | Municipal securities
Fair value of certain assets
Assets measured at fair value	92,201	81,899
Level 2 | U.S. government agencies
Fair value of certain assets
Assets measured at fair value	44,794	19,688
Level 2 | Variable rate demand notes
Fair value of certain assets
Assets measured at fair value	45,373	56,107
Level 3
Fair value of certain assets
Assets measured at fair value	59,727	72,020
Level 3 | Auction rate securities
Fair value of certain assets
Assets measured at fair value	55,636	68,252
Level 3 | Put options related to auction rate securities
Fair value of certain assets
Assets measured at fair value	4,091	3,768
Total fair value
Fair value of certain assets
Cash	52,088	50,202
Assets measured at fair value	733,708	647,448
Total fair value | Money market funds
Fair value of certain assets
Assets measured at fair value	321,065	242,001
Total fair value | U.S. Treasuries
Fair value of certain assets
Assets measured at fair value	65,859	85,521
Total fair value | Certificates of deposit
Fair value of certain assets
Assets measured at fair value	50,559	40,010
Total fair value | Corporate bonds
Fair value of certain assets
Assets measured at fair value	2,042
Total fair value | Municipal securities
Fair value of certain assets
Assets measured at fair value	92,201	81,899
Total fair value | U.S. government agencies
Fair value of certain assets
Assets measured at fair value	44,794	19,688
Total fair value | Variable rate demand notes
Fair value of certain assets
Assets measured at fair value	45,373	56,107
Total fair value | Auction rate securities
Fair value of certain assets
Assets measured at fair value	55,636	68,252
Total fair value | Put options related to auction rate securities
Fair value of certain assets
Assets measured at fair value	4,091	3,768
Put options related to auction rate securities
Fair value of certain assets
Assets measured at fair value	$ 4,100