Investments and Fair Value Measurements Summary of Financial Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Short-term investments:	$ 0	$ 318,202
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets, Fair Value Disclosure	67,890	590,684
Fair Value, Measurements, Recurring | Cash Equivalents | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents:	36,683	236,923
Fair Value, Measurements, Recurring | Short-term Investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Short-term investments:		211,006
Fair Value, Measurements, Recurring | Short-term Investments | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Short-term investments:		14,426
Fair Value, Measurements, Recurring | Short-term Investments | U.S. government treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Short-term investments:		70,650
Fair Value, Measurements, Recurring | Short-term Investments | U.S. government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Short-term investments:		22,101
Fair Value, Measurements, Recurring | Short-term Investments | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Short-term investments:		19
Fair Value, Measurements, Recurring | Prepaid expenses and other current assets: | Current unsecured promissory note
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Other assets:	27,914	25,005
Fair Value, Measurements, Recurring | Prepaid expenses and other current assets: | Israeli Severance Funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Israeli funds	3,293	3,226
Fair Value, Measurements, Recurring | Other Assets | Current unsecured promissory note
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Other assets:		7,328
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets, Fair Value Disclosure	36,683	307,573
Fair Value, Measurements, Recurring | Level 1 | Cash Equivalents | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents:	36,683	236,923
Fair Value, Measurements, Recurring | Level 1 | Short-term Investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Short-term investments:		0
Fair Value, Measurements, Recurring | Level 1 | Short-term Investments | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Short-term investments:		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]
Short-term investments:		70,650
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]
Short-term 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]
Short-term investments:		0
Fair Value, Measurements, Recurring | Level 1 | Prepaid expenses and other current assets: | Current unsecured promissory note
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Other assets:	0	0
Fair Value, Measurements, Recurring | Level 1 | Prepaid expenses and other current assets: | Israeli Severance Funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Israeli funds	0	0
Fair Value, Measurements, Recurring | Level 1 | Other Assets | Current unsecured promissory note
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Other assets:		0
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets, Fair Value Disclosure	3,293	250,778
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 | Short-term Investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Short-term investments:		211,006
Fair Value, Measurements, Recurring | Level 2 | Short-term Investments | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Short-term investments:		14,426
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]
Short-term investments:		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]
Short-term investments:		22,101
Fair Value, Measurements, Recurring | Level 2 | Short-term Investments | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Short-term investments:		19
Fair Value, Measurements, Recurring | Level 2 | Prepaid expenses and other current assets: | Current unsecured promissory note
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Other assets:	0	0
Fair Value, Measurements, Recurring | Level 2 | Prepaid expenses and other current assets: | Israeli Severance Funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Israeli funds	3,293	3,226
Fair Value, Measurements, Recurring | Level 2 | Other Assets | Current unsecured promissory note
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Other assets:		0
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets, Fair Value Disclosure	27,914	32,333
Fair Value, Measurements, Recurring | Level 3 | 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 3 | Short-term Investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Short-term investments:		0
Fair Value, Measurements, Recurring | Level 3 | Short-term Investments | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Short-term investments:		0
Fair Value, Measurements, Recurring | Level 3 | Short-term Investments | U.S. government treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Short-term investments:		0
Fair Value, Measurements, Recurring | Level 3 | Short-term Investments | U.S. government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Short-term investments:		0
Fair Value, Measurements, Recurring | Level 3 | Short-term Investments | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Short-term investments:		0
Fair Value, Measurements, Recurring | Level 3 | Prepaid expenses and other current assets: | Current unsecured promissory note
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Other assets:	27,914	25,005
Fair Value, Measurements, Recurring | Level 3 | Prepaid expenses and other current assets: | Israeli Severance Funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Israeli funds	$ 0	0
Fair Value, Measurements, Recurring | Level 3 | Other Assets | Current unsecured promissory note
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Other assets:		$ 7,328