Investments and Fair Value Measurements Summary of Financial Assets Measured at Fair Value on Recurring Basis (Details) - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	$ 292,011,000	$ 107,572,000
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets, Fair Value Disclosure	508,744,000	552,456,000
Fair Value, Measurements, Recurring | Israeli funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Other assets	3,103,000	3,047,000
Fair Value, Measurements, Recurring | Cash Equivalents | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	181,401,000	431,081,000
Fair Value, Measurements, Recurring | Cash Equivalents | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	22,403,000	4,681,000
Fair Value, Measurements, Recurring | Cash Equivalents | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	4,528,000	3,880,000
Fair Value, Measurements, Recurring | Cash Equivalents | U.S. government treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	5,298,000	2,195,000
Fair Value, Measurements, Recurring | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	292,011,000	98,460,000
Fair Value, Measurements, Recurring | Short-term Investments | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	33,259,000	17,793,000
Fair Value, Measurements, Recurring | Short-term Investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	152,611,000	45,052,000
Fair Value, Measurements, Recurring | Short-term Investments | U.S. government treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	91,875,000	15,291,000
Fair Value, Measurements, Recurring | Short-term Investments | U.S. government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	10,239,000	19,904,000
Fair Value, Measurements, Recurring | Short-term Investments | Foreign bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	4,008,000
Fair Value, Measurements, Recurring | Short-term Investments | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	19,000	420,000
Fair Value, Measurements, Recurring | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments		9,112,000
Fair Value, Measurements, Recurring | Long-term Investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments		4,960,000
Fair Value, Measurements, Recurring | Long-term Investments | U.S. government treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments		2,244,000
Fair Value, Measurements, Recurring | Long-term Investments | U.S. government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments		1,407,000
Fair Value, Measurements, Recurring | Long-term Investments | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments		501,000
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets, Fair Value Disclosure	278,574,000	450,811,000
Fair Value, Measurements, Recurring | Level 1 | Israeli funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Other assets	0	0
Fair Value, Measurements, Recurring | Level 1 | Cash Equivalents | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	181,401,000	431,081,000
Fair Value, Measurements, Recurring | 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 | Level 1 | Cash Equivalents | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	0	0
Fair Value, Measurements, Recurring | Level 1 | Cash Equivalents | U.S. government treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	5,298,000	2,195,000
Fair Value, Measurements, Recurring | 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 | 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 | Level 1 | Short-term Investments | U.S. government treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	91,875,000	15,291,000
Fair Value, Measurements, Recurring | 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 | Level 1 | Short-term Investments | Foreign bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	0
Fair Value, Measurements, Recurring | Level 1 | Short-term Investments | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	0	0
Fair Value, Measurements, Recurring | Level 1 | Long-term Investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments		0
Fair Value, Measurements, Recurring | Level 1 | Long-term Investments | U.S. government treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments		2,244,000
Fair Value, Measurements, Recurring | Level 1 | Long-term Investments | U.S. government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments		0
Fair Value, Measurements, Recurring | Level 1 | Long-term Investments | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments		0
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets, Fair Value Disclosure	230,170,000	101,645,000
Fair Value, Measurements, Recurring | Level 2 | Israeli funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Other assets	3,103,000	3,047,000
Fair Value, Measurements, Recurring | 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 | Level 2 | Cash Equivalents | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	22,403,000	4,681,000
Fair Value, Measurements, Recurring | Level 2 | Cash Equivalents | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	4,528,000	3,880,000
Fair Value, Measurements, Recurring | Level 2 | Cash Equivalents | U.S. government treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	0	0
Fair Value, Measurements, Recurring | Level 2 | Short-term Investments | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	33,259,000	17,793,000
Fair Value, Measurements, Recurring | Level 2 | Short-term Investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	152,611,000	45,052,000
Fair Value, Measurements, Recurring | 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 | Level 2 | Short-term Investments | U.S. government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	10,239,000	19,904,000
Fair Value, Measurements, Recurring | Level 2 | Short-term Investments | Foreign bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	4,008,000
Fair Value, Measurements, Recurring | Level 2 | Short-term Investments | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	$ 19,000	420,000
Fair Value, Measurements, Recurring | Level 2 | Long-term Investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments		4,960,000
Fair Value, Measurements, Recurring | 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 | Level 2 | Long-term Investments | U.S. government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments		1,407,000
Fair Value, Measurements, Recurring | Level 2 | Long-term Investments | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments		$ 501,000