Marketable Securities - Summary of Marketable Securities Held (Detail) - USD ($)	Jun. 30, 2021	Dec. 31, 2020
Amortized cost / Cost
Equity securities	$ 4,305,000	$ 20,017,000
Total	589,706,000	970,180,000
Unrealized gains
Equity securities	0	0
Total	179,000	735,000
Unrealized losses
Equity securities	(1,685,000)	(14,364,000)
Total	(1,725,000)	(14,478,000)
Fair value
Equity securities	2,620,000	5,653,000
Total	588,160,000	956,437,000
Debt securities, available-for-sale, noncurrent	0	0
U.S. government agency securities and treasuries
Amortized cost / Cost
Debt securities	346,331,000	675,043,000
Unrealized gains
Debt securities	138,000	302,000
Unrealized losses
Debt securities	(13,000)	(74,000)
Fair value
Debt securities	346,456,000	675,271,000
Corporate bonds
Amortized cost / Cost
Debt securities	136,133,000	197,171,000
Unrealized gains
Debt securities	41,000	432,000
Unrealized losses
Debt securities	(27,000)	(40,000)
Fair value
Debt securities	136,147,000	197,563,000
Commercial paper
Amortized cost / Cost
Debt securities	102,937,000	77,949,000
Unrealized gains
Debt securities	0	1,000
Unrealized losses
Debt securities	0	0
Fair value
Debt securities	$ 102,937,000	$ 77,950,000