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Summary of Financial Assets Measured at Fair Value on Recurring Basis (Details) (USD $)
Sep. 30, 2014
Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments $ 380,837,000 $ 229,018,000
Fair Value, Measurements, Recurring [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets, Fair Value Disclosure 485,686,000 390,149,000
Fair Value, Measurements, Recurring [Member] | Israeli Severance Funds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Other assets 2,371,000 2,193,000
Fair Value, Measurements, Recurring [Member] | Cash Equivalents | Money market funds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Cash equivalents 74,985,000 143,540,000
Fair Value, Measurements, Recurring [Member] | Cash Equivalents | Commercial paper
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Cash equivalents 27,493,000 15,398,000
Fair Value, Measurements, Recurring [Member] | Short-term Investments
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 244,820,000 127,040,000
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Commercial paper
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 44,840,000 54,328,000
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Corporate bonds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 117,976,000 29,085,000
Fair Value, Measurements, Recurring [Member] | Short-term Investments | U.S. dollar dominated foreign corporate bonds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 913,000 13,971,000
Fair Value, Measurements, Recurring [Member] | Short-term Investments | U.S. government agency bonds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 47,589,000 16,703,000
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Foreign bonds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 913,000  
Fair Value, Measurements, Recurring [Member] | Short-term Investments | U.S. government treasury bonds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 18,520,000  
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Certificates of deposit
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 1,397,000  
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Municipal securities
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 13,585,000 7,014,000
Fair Value, Measurements, Recurring [Member] | Short-term Investments | US Government Agency Bonds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 47,589,000 16,703,000
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Asset-backed securities
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments   5,939,000
Fair Value, Measurements, Recurring [Member] | Long-term Investments
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 136,017,000 101,978,000
Fair Value, Measurements, Recurring [Member] | Long-term Investments | Corporate bonds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 73,403,000 23,313,000
Fair Value, Measurements, Recurring [Member] | Long-term Investments | U.S. dollar dominated foreign corporate bonds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments   19,495,000
Fair Value, Measurements, Recurring [Member] | Long-term Investments | U.S. government agency bonds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 28,888,000 38,118,000
Fair Value, Measurements, Recurring [Member] | Long-term Investments | Foreign bonds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments   19,495,000
Fair Value, Measurements, Recurring [Member] | Long-term Investments | U.S. government treasury bonds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 11,295,000 6,919,000
Fair Value, Measurements, Recurring [Member] | Long-term Investments | Municipal securities
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 10,005,000 8,331,000
Fair Value, Measurements, Recurring [Member] | Long-term Investments | US Government Agency Bonds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments   6,919,000
Fair Value, Measurements, Recurring [Member] | Long-term Investments | Asset-backed securities
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 12,426,000 5,802,000
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1)
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets, Fair Value Disclosure 104,800,000 150,459,000
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Israeli Severance Funds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Other assets 0 0
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash Equivalents | Money market funds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Cash equivalents 74,985,000 143,540,000
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash Equivalents | Commercial paper
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Cash equivalents 0 0
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term Investments | Commercial paper
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 0 0
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term Investments | Corporate bonds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 0 0
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term Investments | U.S. dollar dominated foreign corporate bonds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments   0
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term Investments | Foreign bonds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 0  
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term Investments | U.S. government treasury bonds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 18,520,000  
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term Investments | Certificates of deposit
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 0  
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term Investments | Municipal securities
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 0 0
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term Investments | US Government Agency Bonds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 0 0
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term Investments | Asset-backed securities
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments   0
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Long-term Investments | Corporate bonds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 0 0
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Long-term Investments | U.S. government agency bonds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 0 0
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Long-term Investments | Foreign bonds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments   0
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Long-term Investments | U.S. government treasury bonds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 11,295,000  
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Long-term Investments | Municipal securities
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 0 0
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Long-term Investments | US Government Agency Bonds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments   6,919,000
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Long-term Investments | Asset-backed securities
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 0 0
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2)
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets, Fair Value Disclosure 380,886,000 239,690,000
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Israeli Severance Funds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Other assets 2,371,000 2,193,000
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Cash Equivalents | Money market funds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Cash equivalents 0 0
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Cash Equivalents | Commercial paper
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Cash equivalents 27,493,000 15,398,000
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Short-term Investments | Commercial paper
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 44,840,000 54,328,000
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Short-term Investments | Corporate bonds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 117,976,000 29,085,000
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Short-term Investments | U.S. dollar dominated foreign corporate bonds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments   13,971,000
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Short-term Investments | Foreign bonds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 913,000  
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Short-term Investments | U.S. government treasury bonds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 0  
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Short-term Investments | Certificates of deposit
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 1,397,000  
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Short-term Investments | Municipal securities
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 13,585,000 7,014,000
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Short-term Investments | US Government Agency Bonds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 47,589,000 16,703,000
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Short-term Investments | Asset-backed securities
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments   5,939,000
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Long-term Investments | Corporate bonds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 73,403,000 23,313,000
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Long-term Investments | U.S. government agency bonds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 28,888,000 38,118,000
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Long-term Investments | Foreign bonds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments   19,495,000
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Long-term Investments | U.S. government treasury bonds
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 0  
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Long-term Investments | Municipal securities
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments 10,005,000 8,331,000
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Long-term Investments | US Government Agency Bonds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments   0
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Long-term Investments | Asset-backed securities
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Investments $ 12,426,000 $ 5,802,000