Marketable Securities and Fair Value Measurements - Fair Value Measured On A Recurring Basis (Detail) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, short-term	$ 318,202,000	$ 98,460,000
Marketable securities, long-term	0	9,112,000
Equity method investments	0	45,913,000
Equity securities without readily determinable fair values	5,887,000	9,862,000
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets measured at fair value	590,684,000	552,456,000
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets measured at fair value	307,573,000	450,811,000
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets measured at fair value	250,778,000	101,645,000
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets measured at fair value	32,333,000
Money market funds | Fair Value, Measurements, Recurring | Cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	236,923,000	431,081,000
Money market funds | Fair Value, Measurements, Recurring | Level 1 | Cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	236,923,000	431,081,000
Money market funds | Fair Value, Measurements, Recurring | Level 2 | Cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	0	0
Money market funds | Fair Value, Measurements, Recurring | Level 3 | Cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	0
Corporate bonds | Fair Value, Measurements, Recurring | Cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents		3,880,000
Corporate bonds | Fair Value, Measurements, Recurring | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, short-term	211,006,000	45,052,000
Corporate bonds | Fair Value, Measurements, Recurring | Long Term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, long-term		4,960,000
Corporate bonds | Fair Value, Measurements, Recurring | Level 1 | Cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents		0
Corporate bonds | Fair Value, Measurements, Recurring | Level 1 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, short-term	0	0
Corporate bonds | Fair Value, Measurements, Recurring | Level 1 | Long Term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, long-term		0
Corporate bonds | Fair Value, Measurements, Recurring | Level 2 | Cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents		3,880,000
Corporate bonds | Fair Value, Measurements, Recurring | Level 2 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, short-term	211,006,000	17,793,000
Corporate bonds | Fair Value, Measurements, Recurring | Level 2 | Long Term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, long-term		4,960,000
Corporate bonds | Fair Value, Measurements, Recurring | Level 3 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, short-term	0
Commercial paper | Fair Value, Measurements, Recurring | Cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents		4,681,000
Commercial paper | Fair Value, Measurements, Recurring | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, short-term	14,426,000	17,793,000
Commercial paper | Fair Value, Measurements, Recurring | Level 1 | Cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents		0
Commercial paper | Fair Value, Measurements, Recurring | Level 1 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, short-term	0	0
Commercial paper | Fair Value, Measurements, Recurring | Level 2 | Cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents		4,681,000
Commercial paper | Fair Value, Measurements, Recurring | Level 2 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, short-term	14,426,000	45,052,000
Commercial paper | Fair Value, Measurements, Recurring | Level 3 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, short-term	0
U.S. government agency bonds | Fair Value, Measurements, Recurring | Cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents		2,195,000
U.S. government agency bonds | Fair Value, Measurements, Recurring | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, short-term	22,101,000	19,904,000
U.S. government agency bonds | Fair Value, Measurements, Recurring | Long Term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, long-term		1,407,000
U.S. government agency bonds | Fair Value, Measurements, Recurring | Level 1 | Cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents		2,195,000
U.S. government agency bonds | Fair Value, Measurements, Recurring | Level 1 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, short-term	0	0
U.S. government agency bonds | Fair Value, Measurements, Recurring | Level 1 | Long Term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, long-term		0
U.S. government agency bonds | Fair Value, Measurements, Recurring | Level 2 | Cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents		0
U.S. government agency bonds | Fair Value, Measurements, Recurring | Level 2 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, short-term	22,101,000	19,904,000
U.S. government agency bonds | Fair Value, Measurements, Recurring | Level 2 | Long Term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, long-term		1,407,000
U.S. government agency bonds | Fair Value, Measurements, Recurring | Level 3 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, short-term	0
U.S. Government Treasury Bonds | Fair Value, Measurements, Recurring | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, short-term	70,650,000	15,291,000
U.S. Government Treasury Bonds | Fair Value, Measurements, Recurring | Long Term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, long-term		2,244,000
U.S. Government Treasury Bonds | Fair Value, Measurements, Recurring | Level 1 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, short-term	70,650,000	15,291,000
U.S. Government Treasury Bonds | Fair Value, Measurements, Recurring | Level 2 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, short-term	0	0
U.S. Government Treasury Bonds | Fair Value, Measurements, Recurring | Level 3 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, short-term	0
Certificates of Deposit | Fair Value, Measurements, Recurring | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, short-term	19,000	420,000
Certificates of Deposit | Fair Value, Measurements, Recurring | Long Term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, long-term		501,000
Certificates of Deposit | Fair Value, Measurements, Recurring | Level 1 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, short-term	0	0
Certificates of Deposit | Fair Value, Measurements, Recurring | Level 2 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, short-term	19,000	420,000
Certificates of Deposit | Fair Value, Measurements, Recurring | Level 2 | Long Term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, long-term		501,000
Certificates of Deposit | Fair Value, Measurements, Recurring | Level 3 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities, short-term	0
Israeli funds | Fair Value, Measurements, Recurring | Prepaid expenses and other current assets:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Israeli funds	3,226,000
Israeli funds | Fair Value, Measurements, Recurring | Other Assets:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Israeli funds		3,047,000
Israeli funds | Fair Value, Measurements, Recurring | Level 1 | Prepaid expenses and other current assets:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Israeli funds	0
Israeli funds | Fair Value, Measurements, Recurring | Level 1 | Other Assets:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Israeli funds		0
Israeli funds | Fair Value, Measurements, Recurring | Level 2 | Prepaid expenses and other current assets:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Israeli funds	3,226,000
Israeli funds | Fair Value, Measurements, Recurring | Level 2 | Other Assets:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Israeli funds		3,047,000
Israeli funds | Fair Value, Measurements, Recurring | Level 3 | Prepaid expenses and other current assets:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Israeli funds	0
Unsecured promissory note | Fair Value, Measurements, Recurring | Prepaid expenses and other current assets:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Current unsecured promissory note	25,005,000
Unsecured promissory note | Fair Value, Measurements, Recurring | Other Assets:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Long term unsecured promissory note	7,328,000
Unsecured promissory note | Fair Value, Measurements, Recurring | Level 1 | Prepaid expenses and other current assets:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Current unsecured promissory note	0
Unsecured promissory note | Fair Value, Measurements, Recurring | Level 1 | Other Assets:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Long term unsecured promissory note	0
Unsecured promissory note | Fair Value, Measurements, Recurring | Level 2 | Prepaid expenses and other current assets:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Current unsecured promissory note	0
Unsecured promissory note | Fair Value, Measurements, Recurring | Level 2 | Other Assets:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Long term unsecured promissory note	0
Unsecured promissory note | Fair Value, Measurements, Recurring | Level 3 | Prepaid expenses and other current assets:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Current unsecured promissory note	25,005,000
Unsecured promissory note | Fair Value, Measurements, Recurring | Level 3 | Other Assets:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Long term unsecured promissory note	$ 7,328,000
Equity securities without readily determinable fair values | Fair Value, Measurements, Recurring | Other Assets:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Equity method investments
Equity securities without readily determinable fair values
Equity securities without readily determinable fair values | Fair Value, Measurements, Recurring | Level 1 | Other Assets:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Equity method investments		0
Equity securities without readily determinable fair values		0
Equity securities without readily determinable fair values | Fair Value, Measurements, Recurring | Level 3 | Other Assets:
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Equity method investments
Equity securities without readily determinable fair values