Investments, Available-for-Sale (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Short-term investments, available-for-sale
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	$ 326,152	$ 884,038
Gross unrealized gains	0	562
Gross unrealized losses	(255)	(103)
Estimated fair value	325,897	884,497
Short-term investments, available-for-sale \| US government and government agency notes
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	89,011	621,281
Gross unrealized gains	0	178
Gross unrealized losses	(37)	(59)
Estimated fair value	88,974	621,400
Short-term investments, available-for-sale \| Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	122,408	160,244
Gross unrealized gains	0	362
Gross unrealized losses	(186)	(38)
Estimated fair value	122,222	160,568
Short-term investments, available-for-sale \| Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	114,733	102,513
Gross unrealized gains	0	22
Gross unrealized losses	(32)	(6)
Estimated fair value	114,701	$ 102,529
Investments, available-for-sale
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	161,252
Gross unrealized gains	0
Gross unrealized losses	(456)
Estimated fair value	160,796
Investments, available-for-sale \| US government and government agency notes
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	106,501
Gross unrealized gains	0
Gross unrealized losses	(291)
Estimated fair value	106,210
Investments, available-for-sale \| Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	54,751
Gross unrealized gains	0
Gross unrealized losses	(165)
Estimated fair value	$ 54,586

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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