INVESTMENTS (Details 2) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Disclosure of underlying contractual maturities of investments
Amortized Cost	$ 311,420	$ 291,246
Fair Value	311,400	288,813
Less than 1 year | U.S. Treasuries
Disclosure of underlying contractual maturities of investments
Amortized Cost	60,859	66,521
Fair Value	60,868	66,519
Less than 1 year | Certificates of deposit
Disclosure of underlying contractual maturities of investments
Amortized Cost	19,530	7,004
Fair Value	19,530	7,004
Less than 1 year | Corporate bonds
Disclosure of underlying contractual maturities of investments
Amortized Cost	2,042
Fair Value	2,042
Less than 1 year | Municipal securities
Disclosure of underlying contractual maturities of investments
Amortized Cost	83,186	71,266
Fair Value	83,162	71,251
U.S. government agency securities | Less than 1 year
Disclosure of underlying contractual maturities of investments
Amortized Cost	44,794	19,688
Fair Value	44,789	19,680
Due 1 - 10 years | Variable rate demand notes
Disclosure of underlying contractual maturities of investments
Amortized Cost	6,175	3,001
Fair Value	6,175	3,001
Due 11 - 20 years | Variable rate demand notes
Disclosure of underlying contractual maturities of investments
Amortized Cost	12,801	11,002
Fair Value	12,801	11,002
Due 21 - 30 years | Variable rate demand notes
Disclosure of underlying contractual maturities of investments
Amortized Cost	19,221	30,426
Fair Value	19,221	30,426
Due 31 - 40 years | Variable rate demand notes
Disclosure of underlying contractual maturities of investments
Amortized Cost	7,176	11,678
Fair Value	7,176	11,678
Due 11 - 20 years | Auction rate securities
Disclosure of underlying contractual maturities of investments
Amortized Cost	9,786	10,305
Fair Value	9,786	9,819
Due 21 - 30 years | Auction rate securities
Disclosure of underlying contractual maturities of investments
Amortized Cost	40,249	48,779
Fair Value	40,249	46,857
Due 31 - 40 years | Auction rate securities
Disclosure of underlying contractual maturities of investments
Amortized Cost	5,601	11,576
Fair Value	$ 5,601	$ 11,576