Quarterly report pursuant to Section 13 or 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
9 Months Ended 12 Months Ended
Sep. 30, 2011
Dec. 31, 2010
Cash and cash equivalents $ 451,657 $ 433,914
U.S. Treasury securities and obligations of U.S. government corporations and agencies 22,344 21,905
Corporate securities 192,448 200,115
Municipal securities, General obligation 120,654 113,461
Municipal securities, Pre-refunded 33,295 33,198
Municipal securities, Revenue 117,427 102,964
Municipal securities, Variable rate demand notes 108,723 106,540
Asset backed securities 19,206 17,591
Total investments 614,097 595,774
Cash and cash equivalents 13,488 8,814
Certificates of deposit 5,890 6,814
U.S. Treasury securities and obligations of U.S. government corporations and agencies 7,390 7,130
Total restricted deposits 26,768 22,758
Interest rate swap contract 10,489  
Total assets at fair value 1,103,011 1,052,446
Level I [Member]
   
Cash and cash equivalents 451,657 433,914
U.S. Treasury securities and obligations of U.S. government corporations and agencies 17,019 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 17,019 14,809
Cash and cash equivalents 13,488 8,814
Certificates of deposit 5,890 6,814
U.S. Treasury securities and obligations of U.S. government corporations and agencies 7,390 7,130
Total restricted deposits 26,768 22,758
Interest rate swap contract    
Total assets at fair value 495,444 471,481
Level II [Member]
   
Cash and cash equivalents    
U.S. Treasury securities and obligations of U.S. government corporations and agencies 5,325 7,096
Corporate securities 192,448 200,115
Municipal securities, General obligation 120,654 113,461
Municipal securities, Pre-refunded 33,295 33,198
Municipal securities, Revenue 117,427 102,964
Municipal securities, Variable rate demand notes 108,723 106,540
Asset backed securities 19,206 17,591
Total investments 597,078 580,965
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 10,489  
Total assets at fair value 607,567 580,965
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