Marketable Securities - Summary of Marketable Securities Held (Detail) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Amortized cost / Cost
Equity securities	$ 20,017,000	$ 20,017,000
Marketable securities	1,128,582,000	915,152,000
Unrealized gains
Equity securities	0	0
Marketable securities	1,620,000	2,838,000
Unrealized losses
Equity securities	(16,215,000)	(7,147,000)
Marketable securities	(16,496,000)	(7,238,000)
Fair value
Equity securities	3,802,000	12,870,000
Marketable securities:	1,113,706,000	910,752,000
Debt securities, available-for-sale, noncurrent	0	0
U.S. government agency securities and treasuries
Amortized cost / Cost
Debt securities	778,742,000	633,970,000
Unrealized gains
Debt securities	539,000	2,014,000
Unrealized losses
Debt securities	(223,000)	(48,000)
Fair value
Debt securities	779,058,000	635,936,000
Certificates of deposit
Amortized cost / Cost
Debt securities		960,000
Unrealized gains
Debt securities		0
Unrealized losses
Debt securities		0
Fair value
Debt securities		960,000
Corporate bonds
Amortized cost / Cost
Debt securities	237,884,000	185,827,000
Unrealized gains
Debt securities	1,079,000	824,000
Unrealized losses
Debt securities	(58,000)	(43,000)
Fair value
Debt securities	238,905,000	186,608,000
Commercial paper
Amortized cost / Cost
Debt securities	91,939,000	74,378,000
Unrealized gains
Debt securities	2,000	0
Unrealized losses
Debt securities	0	0
Fair value
Debt securities	$ 91,941,000	$ 74,378,000

Equity securities unrealized gain.

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Equity securities unrealized loss.

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Marketable securities amortized cost.

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), classified as noncurrent.

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