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INVESTMENTS (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2011
Dec. 31, 2010
Held to Maturity and Available-for-sale    
Held to maturity and available-for-sale securities, Amortized Cost $ 401,944 $ 248,376
Held to maturity and available-for-sale securities, Gross Unrealized Holding Gains 8  
Held to maturity and available-for-sale securities, Gross Unrealized Holding Losses 64 2,433
Held to maturity and available-for-sale securities, Fair Value 401,888 245,943
Held to maturity and available-for-sale securities, Continuous Unrealized Loss Position less than 12 Months 64 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 434,420 288,813
Short-term | U.S. Treasuries
   
Held to Maturity    
Amortized Cost 8,034 66,521
Gross Unrealized Holding Gains 5  
Gross Unrealized Holding Losses   2
Fair Value 8,039 66,519
Continuous Unrealized Loss Position less than 12 Months   2
Short-term | Certificates of deposit
   
Held to Maturity    
Amortized Cost 29,034 7,004
Gross Unrealized Holding Gains 1  
Fair Value 29,035 7,004
Short-term | Corporate bonds
   
Held to Maturity    
Amortized Cost 2,022  
Fair Value 2,022  
Short-term | Municipal securities
   
Held to Maturity    
Amortized Cost 284,605 71,266
Gross Unrealized Holding Losses 64 15
Fair Value 284,541 71,251
Continuous Unrealized Loss Position less than 12 Months 64 15
Short-term | U.S. government agency securities.
   
Held to Maturity    
Amortized Cost 16,005 19,688
Gross Unrealized Holding Gains 2  
Gross Unrealized Holding Losses   8
Fair Value 16,007 19,680
Continuous Unrealized Loss Position less than 12 Months   8
Short-term | Variable rate demand notes
   
Available-for-sale    
Amortized Cost 58,924 56,107
Fair Value 58,924 56,107
Short-term | Auction rate securities
   
Trading    
Fair Value 12,658 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 $ 19,874 $ 18,807