INVESTMENTS (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Held to Maturity and Available-for-sale
Held to maturity and available-for-sale securities, Amortized Cost	$ 259,104	$ 248,376
Held to maturity and available-for-sale securities, Gross Unrealized Holding Gains	9
Held to maturity and available-for-sale securities, Gross Unrealized Holding Losses	29	2,433
Held to maturity and available-for-sale securities, Fair Value	259,084	245,943
Held to maturity and available-for-sale securities, Continuous Unrealized Loss Position less than 12 Months	29	25
Held to maturity and available-for-sale securities, Continuous Unrealized Loss Position greater than 12 Months		2,408
Held to Maturity, Available-for-sale, and Trading
Total Fair Value	311,400	288,813
Short-term | U.S. Treasuries
Held to Maturity
Amortized Cost	60,859	66,521
Gross Unrealized Holding Gains	9
Gross Unrealized Holding Losses		2
Fair Value	60,868	66,519
Continuous Unrealized Loss Position less than 12 Months		2
Short-term | Certificates of deposit
Held to Maturity
Amortized Cost	19,530	7,004
Fair Value	19,530	7,004
Short-term | Corporate bonds
Held to Maturity
Amortized Cost	2,042
Fair Value	2,042
Short-term | Municipal securities
Held to Maturity
Amortized Cost	83,186	71,266
Gross Unrealized Holding Losses	24	15
Fair Value	83,162	71,251
Continuous Unrealized Loss Position less than 12 Months	24	15
Short-term | U.S. government agencies
Held to Maturity
Amortized Cost	44,794	19,688
Gross Unrealized Holding Losses	5	8
Fair Value	44,789	19,680
Continuous Unrealized Loss Position less than 12 Months	5	8
Short-term | Variable rate demand notes
Available-for-sale
Amortized Cost	45,373	56,107
Fair Value	45,373	56,107
Short-term | Auction rate securities
Trading
Fair Value	25,434	24,063
Long-term | Auction rate securities
Available-for-sale
Amortized Cost	3,320	27,790
Gross Unrealized Holding Losses		2,408
Fair Value	3,320	25,382
Continuous Unrealized Loss Position greater than 12 Months		2,408
Trading
Fair Value	$ 26,882	$ 18,807