Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements (Fair Value Measurements By Level For Assets Measured At Fair Value On A Recurring Basis) (Details)

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Fair Value Measurements (Fair Value Measurements By Level For Assets Measured At Fair Value On A Recurring Basis) (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Cash and cash equivalents $ 573,698 $ 434,166
U.S. Treasury securities and obligations of U.S. government corporations and agencies 22,486 21,905
Corporate securities 189,029 200,115
Municipal securities, General obligation 129,608 113,461
Municipal securities, Pre-refunded 33,712 33,198
Municipal securities, Revenue 120,860 102,964
Municipal securities, Variable rate demand notes 64,658 106,540
Asset backed securities 52,192 17,591
Total investments 612,545 595,774
Cash and cash equivalents 13,775 8,814
Certificates of deposit 5,890 6,814
U.S. Treasury securities and obligations of U.S. government corporations and agencies 7,153 7,130
Total restricted deposits 26,818 22,758
Interest rate swap contract 11,431  
Total assets at fair value 1,224,492 1,052,698
Level I [Member]
   
Cash and cash equivalents 573,698 434,166
U.S. Treasury securities and obligations of U.S. government corporations and agencies 17,091 14,809
Corporate securities      
Municipal securities, General obligation      
Municipal securities, Pre-refunded      
Municipal securities, Revenue      
Municipal securities, Variable rate demand notes      
Asset backed securities      
Total investments 17,091 14,809
Cash and cash equivalents 13,775 8,814
Certificates of deposit 5,890 6,814
U.S. Treasury securities and obligations of U.S. government corporations and agencies 7,153 7,130
Total restricted deposits 26,818 22,758
Interest rate swap contract     
Total assets at fair value 617,607 471,733
Level II [Member]
   
Cash and cash equivalents      
U.S. Treasury securities and obligations of U.S. government corporations and agencies 5,395 7,096
Corporate securities 189,029 200,115
Municipal securities, General obligation 129,608 113,461
Municipal securities, Pre-refunded 33,712 33,198
Municipal securities, Revenue 120,860 102,964
Municipal securities, Variable rate demand notes 64,658 106,540
Asset backed securities 52,192 17,591
Total investments 595,454 580,965
Cash and cash equivalents      
Certificates of deposit      
U.S. Treasury securities and obligations of U.S. government corporations and agencies      
Total restricted deposits      
Interest rate swap contract 11,431  
Total assets at fair value 606,885 580,965
Level III [Member]
   
Cash and cash equivalents      
U.S. Treasury securities and obligations of U.S. government corporations and agencies      
Corporate securities      
Municipal securities, General obligation      
Municipal securities, Pre-refunded      
Municipal securities, Revenue      
Municipal securities, Variable rate demand notes      
Asset backed securities      
Total investments      
Cash and cash equivalents      
Certificates of deposit      
U.S. Treasury securities and obligations of U.S. government corporations and agencies      
Total restricted deposits      
Interest rate swap contract     
Total assets at fair value