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Fair Value Of Financial Instruments (Fair Value, Assets And Liabilities Measured On Recurring Basis) (Details) - Measured On A Recurring Basis [Member] - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Total fair value of assets $ 1,756,854 $ 1,763,970
Total fair of value of liabilities 380 14,426
Embedded Derivative Financial Instruments [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Derivative Financial Instruments, Liabilities, Fair Value Disclosure   14,339
Money Market Funds [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Investments in money market funds 192,524 134,790
Debt Securities Issued By The U.S. Treasury And Other U.S. Government Corporations And Agencies [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Investments, Fair Value Disclosure 1,561,918 1,626,764
Equity Securities [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Investments, Fair Value Disclosure 2,372 2,174
Forward Contracts [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Foreign currency forward contracts assets 40 242
Foreign currency forward contracts liabilities 380 87
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Total fair value of assets 1,756,814 1,763,728
Total fair of value of liabilities 0 0
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Embedded Derivative Financial Instruments [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Derivative Financial Instruments, Liabilities, Fair Value Disclosure   0
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Money Market Funds [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Investments in money market funds 192,524 134,790
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Debt Securities Issued By The U.S. Treasury And Other U.S. Government Corporations And Agencies [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Investments, Fair Value Disclosure   1,626,764
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Equity Securities [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Investments, Fair Value Disclosure 2,372 2,174
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Forward Contracts [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Foreign currency forward contracts assets 0 0
Foreign currency forward contracts liabilities 0 0
Significant Other Observable Inputs (Level 2) [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Total fair value of assets 40 242
Total fair of value of liabilities 380 87
Significant Other Observable Inputs (Level 2) [Member] | Embedded Derivative Financial Instruments [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Derivative Financial Instruments, Liabilities, Fair Value Disclosure   0
Significant Other Observable Inputs (Level 2) [Member] | Money Market Funds [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Investments in money market funds 0 0
Significant Other Observable Inputs (Level 2) [Member] | Debt Securities Issued By The U.S. Treasury And Other U.S. Government Corporations And Agencies [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Investments, Fair Value Disclosure 0 0
Significant Other Observable Inputs (Level 2) [Member] | Equity Securities [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Investments, Fair Value Disclosure 0 0
Significant Other Observable Inputs (Level 2) [Member] | Forward Contracts [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Foreign currency forward contracts assets 40 242
Foreign currency forward contracts liabilities 380 87
Significant Unobservable Inputs (Level 3) [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Total fair value of assets 0 0
Total fair of value of liabilities 0 14,339
Significant Unobservable Inputs (Level 3) [Member] | Embedded Derivative Financial Instruments [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Derivative Financial Instruments, Liabilities, Fair Value Disclosure   14,339
Significant Unobservable Inputs (Level 3) [Member] | Money Market Funds [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Investments in money market funds 0 0
Significant Unobservable Inputs (Level 3) [Member] | Debt Securities Issued By The U.S. Treasury And Other U.S. Government Corporations And Agencies [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Investments, Fair Value Disclosure 0 0
Significant Unobservable Inputs (Level 3) [Member] | Forward Contracts [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Foreign currency forward contracts assets 0 0
Foreign currency forward contracts liabilities $ 0 $ 0