Marketable Securities - Summary of Marketable Securities Held (Detail) - USD ($)	Mar. 31, 2021	Dec. 31, 2020
Amortized cost / Cost
Equity securities	$ 4,305,000	$ 20,017,000
Marketable securities	655,072,000	970,180,000
Unrealized gains
Equity securities	0	0
Marketable securities	419,000	735,000
Unrealized losses
Equity securities	(1,598,000)	(14,364,000)
Marketable securities	(1,654,000)	(14,478,000)
Fair value
Equity securities	2,707,000	5,653,000
Marketable securities:	653,837,000	956,437,000
Debt securities, available-for-sale, noncurrent	0	0
U.S. government agency securities and treasuries
Amortized cost / Cost
Debt securities	429,039,000	675,043,000
Unrealized gains
Debt securities	256,000	302,000
Unrealized losses
Debt securities	(16,000)	(74,000)
Fair value
Debt securities	429,279,000	675,271,000
Corporate bonds
Amortized cost / Cost
Debt securities	142,773,000	197,171,000
Unrealized gains
Debt securities	163,000	432,000
Unrealized losses
Debt securities	(40,000)	(40,000)
Fair value
Debt securities	142,896,000	197,563,000
Commercial paper
Amortized cost / Cost
Debt securities	78,955,000	77,949,000
Unrealized gains
Debt securities	0	1,000
Unrealized losses
Debt securities	0	0
Fair value
Debt securities	$ 78,955,000	$ 77,950,000