Summary of Company Cash, Cash Equivalents and Short-term Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents, Amortized cost	$ 1,474,639	$ 835,044
Cash and cash equivalents, Gross unrealized gains	2	36
Cash and cash equivalents, Gross unrealized losses	(4)
Cash and cash equivalents	1,474,637	835,080	$ 724,829
Cash, cash equivalents and investments, Amortized cost	1,816,091	1,124,679
Cash, cash equivalents and investments, Gross unrealized gains	17	71
Cash, cash equivalents and investments, Gross unrealized losses	(4)	(2)
Cash, cash equivalents and investments, Fair value	1,816,104	1,124,748
Available for sale debt securities current, Amortized cost	341,452	289,635
Available for sale debt securities current, Gross unrealized gains	15	35
Available for sale debt securities current, Gross unrealized losses		(2)
Available for sale debt securities current, Fair value	341,467	289,668
Cash and Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents, Amortized cost	923,490	315,589
Cash and cash equivalents	923,490	315,589
Government and Government Agency Bonds [Member]
Cash and Cash Equivalents [Line Items]
Available for sale debt securities, Amortized cost	892,601	809,090
Available for sale debt securities, Gross unrealized gains	17	71
Available for sale debt securities, Gross unrealized losses	(4)	(2)
Available for sale debt securities, Fair value	$ 892,614	$ 809,159

Available for sale debt securities current accumulated gross unrealized gain before tax.

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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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