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Financial Instruments - Summary of Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Short-term, Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments (available for sale), Cost	$ 138	$ 139
Total short-term investments (available for sale), Gross Unrealized Gain		1
Total short-term investments (available for sale), Fair value	138	140
Short-term, Non-governmental Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments (available for sale), Cost	386,509	288,578
Total short-term investments (available for sale), Gross Unrealized Gain	81	331
Total short-term investments (available for sale), Gross Unrealized (Loss)	(164)	(176)
Total short-term investments (available for sale), Fair value	386,426	288,733
Short-term, Treasury and Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments (available for sale), Cost	2,746	11,799
Total short-term investments (available for sale), Gross Unrealized Gain	1	6
Total short-term investments (available for sale), Gross Unrealized (Loss)	(1)	(2)
Total short-term investments (available for sale), Fair value	2,746	11,803
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments (available for sale), Cost	389,393	300,516
Total short-term investments (available for sale), Gross Unrealized Gain	82	338
Total short-term investments (available for sale), Gross Unrealized (Loss)	(165)	(178)
Total short-term investments (available for sale), Fair value	$ 389,310	$ 300,676

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