INVESTMENTS (Details 2) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Disclosure of underlying contractual maturities of investments
Amortized Cost	$ 434,476	$ 291,246
Fair Value	434,420	288,813
U.S. Treasuries | Less than 1 year
Disclosure of underlying contractual maturities of investments
Amortized Cost	8,034	66,521
Fair Value	8,039	66,519
Certificates of deposit | Less than 1 year
Disclosure of underlying contractual maturities of investments
Amortized Cost	29,034	7,004
Fair Value	29,035	7,004
Corporate bonds | Less than 1 year
Disclosure of underlying contractual maturities of investments
Amortized Cost	2,022
Fair Value	2,022
Municipal securities | Less than 1 year
Disclosure of underlying contractual maturities of investments
Amortized Cost	284,605	71,266
Fair Value	284,541	71,251
U.S. government agency securities | Less than 1 year
Disclosure of underlying contractual maturities of investments
Amortized Cost	16,005	19,688
Fair Value	16,007	19,680
Variable rate demand notes | Due 1 - 10 years
Disclosure of underlying contractual maturities of investments
Amortized Cost	5,775	3,001
Fair Value	5,775	3,001
Variable rate demand notes | Due 11 - 20 years
Disclosure of underlying contractual maturities of investments
Amortized Cost	12,716	11,002
Fair Value	12,716	11,002
Variable rate demand notes | Due 21 - 30 years
Disclosure of underlying contractual maturities of investments
Amortized Cost	27,902	30,426
Fair Value	27,902	30,426
Variable rate demand notes | Due 31 - 40 years
Disclosure of underlying contractual maturities of investments
Amortized Cost	12,532	11,678
Fair Value	12,532	11,678
Auction rate securities | Due 11 - 20 years
Disclosure of underlying contractual maturities of investments
Amortized Cost	5,158	10,305
Fair Value	5,158	9,819
Auction rate securities | Due 21 - 30 years
Disclosure of underlying contractual maturities of investments
Amortized Cost	25,134	48,779
Fair Value	25,134	46,857
Auction rate securities | Due 31 - 40 years
Disclosure of underlying contractual maturities of investments
Amortized Cost	5,559	11,576
Fair Value	$ 5,559	$ 11,576