Quarterly report pursuant to Section 13 or 15(d)

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

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Fair Value Measurements (Fair Value Measurements By Level For Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2014
Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and Cash Equivalents $ 1,199,784 $ 974,304
U.S. Treasury securities and obligations of U.S. government corporations and agencies 307,735 224,423
Corporate securities 399,348 296,086
Municipal securities, General obligation 39,408 54,422
Municipal securities, Pre-refunded 8,535 10,917
Municipal securities, Revenue 69,400 69,054
Municipal securities, Variable rate demand notes 18,000 28,575
Asset backed securities 179,985 139,050
Mortgage backed securities 48,270 33,891
Total investments 1,070,681 856,418
Cash and cash equivalents 57,608 26,642
Certificates of deposit 5,892 5,891
U.S. Treasury securities and obligations of U.S. government corporations and agencies 14,942 14,413
Total restricted deposits 78,442 46,946
Other long-term assets: Interest rate swap agreements 9,506 9,576
Total assets at fair value 2,358,413 1,887,244
Level I [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and Cash Equivalents 1,199,784 974,304
U.S. Treasury securities and obligations of U.S. government corporations and agencies 288,546 212,185
Corporate securities 0 0
Municipal securities, General obligation 0 0
Municipal securities, Pre-refunded 0 0
Municipal securities, Revenue 0 0
Municipal securities, Variable rate demand notes 0 0
Asset backed securities 0 0
Mortgage backed securities 0 0
Total investments 288,546 212,185
Cash and cash equivalents 57,608 26,642
Certificates of deposit 5,892 5,891
U.S. Treasury securities and obligations of U.S. government corporations and agencies 14,942 14,413
Total restricted deposits 78,442 46,946
Other long-term assets: Interest rate swap agreements 0 0
Total assets at fair value 1,566,772 1,233,435
Level II [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and Cash Equivalents 0 0
U.S. Treasury securities and obligations of U.S. government corporations and agencies 19,189 12,238
Corporate securities 399,348 296,086
Municipal securities, General obligation 39,408 54,422
Municipal securities, Pre-refunded 8,535 10,917
Municipal securities, Revenue 69,400 69,054
Municipal securities, Variable rate demand notes 18,000 28,575
Asset backed securities 179,985 139,050
Mortgage backed securities 48,270 33,891
Total investments 782,135 644,233
Cash and cash equivalents 0 0
Certificates of deposit 0 0
U.S. Treasury securities and obligations of U.S. government corporations and agencies 0 0
Total restricted deposits 0 0
Other long-term assets: Interest rate swap agreements 9,506 9,576
Total assets at fair value 791,641 653,809
Level III [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and Cash Equivalents 0 0
U.S. Treasury securities and obligations of U.S. government corporations and agencies 0 0
Corporate securities 0 0
Municipal securities, General obligation 0 0
Municipal securities, Pre-refunded 0 0
Municipal securities, Revenue 0 0
Municipal securities, Variable rate demand notes 0 0
Asset backed securities 0 0
Mortgage backed securities 0 0
Total investments 0 0
Cash and cash equivalents 0 0
Certificates of deposit 0 0
U.S. Treasury securities and obligations of U.S. government corporations and agencies 0 0
Total restricted deposits 0 0
Other long-term assets: Interest rate swap agreements 0 0
Total assets at fair value $ 0 $ 0