Summary of Financial Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	$ 255,838	$ 510,951
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Long-term notes receivable	2,000
Assets, Fair Value Disclosure	353,296	624,646
Fair Value, Measurements, Recurring [Member] | Israeli Severance Funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Other assets	2,527	2,436
Fair Value, Measurements, Recurring [Member] | Cash Equivalents | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	92,931	70,148
Fair Value, Measurements, Recurring [Member] | Cash Equivalents | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents		36,887
Fair Value, Measurements, Recurring [Member] | Cash Equivalents | U.S. government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents		3,599
Fair Value, Measurements, Recurring [Member] | Cash Equivalents | U.S. dollar dominated foreign corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents		625
Fair Value, Measurements, Recurring [Member] | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	193,018	359,581
Amortized Cost	192,978	359,956
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	19,029	38,537
Amortized Cost	19,029	38,537
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	81,944	179,520
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		508
Amortized Cost		510
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	30,611	75,119
Amortized Cost	30,587	75,172
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	47,733	51,683
Amortized Cost	47,702	51,763
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	4,500
Amortized Cost	4,500
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Municipal securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	9,201	14,214
Amortized Cost	9,205	14,209
Fair Value, Measurements, Recurring [Member] | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	62,820	151,370
Amortized Cost	62,677	151,869
Fair Value, Measurements, Recurring [Member] | Long-term Investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	43,212	63,803
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	9,260	43,675
Amortized Cost	9,249	43,853
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	10,048	37,566
Amortized Cost	10,027	37,673
Fair Value, Measurements, Recurring [Member] | Long-term Investments | Municipal securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments		3,991
Amortized Cost		3,993
Fair Value, Measurements, Recurring [Member] | Long-term Investments | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	300	2,335
Amortized Cost	300	2,338
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Long-term notes receivable	2,000
Assets, Fair Value Disclosure	2,000
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]
Long-term notes receivable	0
Assets, Fair Value Disclosure	150,712	159,397
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	92,931	70,148
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
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash Equivalents | U.S. government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents		0
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash Equivalents | U.S. dollar dominated foreign corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents		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 | 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) | Short-term Investments | U.S. government treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	47,733	51,683
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) | 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 | U.S. government treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	10,048	37,566
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
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]
Long-term notes receivable	0
Assets, Fair Value Disclosure	200,584	465,249
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	2,527	2,436
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		36,887
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Cash Equivalents | U.S. government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents		3,599
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Cash Equivalents | U.S. dollar dominated foreign corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents		625
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	19,029	38,537
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	81,944	179,520
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		508
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Short-term Investments | U.S. government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	30,611	75,119
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	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	4,500
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	9,201	14,214
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	43,212	63,803
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	9,260	43,675
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	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		3,991
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	$ 300	$ 2,335