Summary of Financial Assets Measured at Fair Value on Recurring Basis (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	$ 224,576,000	$ 49,737,000
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets, Fair Value Disclosure	317,729,000	139,071,000
Fair Value, Measurements, Recurring [Member] | Israeli Severance Funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Other assets	2,422,000	2,218,000
Fair Value, Measurements, Recurring [Member] | Cash Equivalents | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	77,484,000	86,166,000
Fair Value, Measurements, Recurring [Member] | Cash Equivalents | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	13,247,000	950,000
Fair Value, Measurements, Recurring [Member] | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	147,740,000	28,485,000
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	57,921,000	4,647,000
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	32,449,000	18,770,000
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	37,487,000	5,068,000
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Foreign bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	15,730,000
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	1,149,000
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	37,487,000
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	3,004,000
Fair Value, Measurements, Recurring [Member] | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	76,836,000	21,252,000
Fair Value, Measurements, Recurring [Member] | Long-term Investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	19,822,000	16,141,000
Fair Value, Measurements, Recurring [Member] | Long-term Investments | Agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	6,153,000	2,070,000
Fair Value, Measurements, Recurring [Member] | Long-term Investments | Foreign bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	16,179,000	3,041,000
Fair Value, Measurements, Recurring [Member] | Long-term Investments | US Treasury and Government [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	11,278,000
Fair Value, Measurements, Recurring [Member] | Long-term Investments | Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	14,257,000
Fair Value, Measurements, Recurring [Member] | Long-term Investments | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	9,147,000
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	88,762,000	86,166,000
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
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	77,484,000	86,166,000
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 | 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 | Municipal Securities [Member]
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 | US Government Agency Bonds [Member]
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 | Asset-backed Securities [Member]
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 | 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	0
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Long-term Investments | US Treasury and Government [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	11,278,000
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Long-term Investments | Municipal Securities [Member]
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 | Asset-backed Securities [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)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets, Fair Value Disclosure	228,967,000	52,905,000
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	2,422,000	2,218,000
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	13,247,000	950,000
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	57,921,000	4,647,000
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	32,449,000	18,770,000
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Short-term Investments | Agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments		5,068,000
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	15,730,000
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Short-term Investments | Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	1,149,000
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	37,487,000
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Short-term Investments | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	3,004,000
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	19,822,000	16,141,000
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Long-term Investments | Agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	6,153,000	2,070,000
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	16,179,000	3,041,000
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Long-term Investments | US Treasury and Government [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 | Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	14,257,000
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Long-term Investments | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	$ 9,147,000