Financial Instruments - Summary of Investments (Detail) - USD ($) $ in Thousands	Sep. 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	$ 5,057	$ 139
Total short-term investments (available for sale), Gross Unrealized Gain		1
Total short-term investments (available for sale), Gross Unrealized (Loss)	(3)
Total short-term investments (available for sale), Fair value	5,054	140
Short-term, Non-governmental Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments (available for sale), Cost	203,643	288,578
Total short-term investments (available for sale), Gross Unrealized Gain	38	331
Total short-term investments (available for sale), Gross Unrealized (Loss)	(65)	(176)
Total short-term investments (available for sale), Fair value	203,616	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	723	11,799
Total short-term investments (available for sale), Gross Unrealized Gain	1	6
Total short-term investments (available for sale), Gross Unrealized (Loss)	(3)	(2)
Total short-term investments (available for sale), Fair value	721	11,803
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments (available for sale), Cost	209,423	300,516
Total short-term investments (available for sale), Gross Unrealized Gain	39	338
Total short-term investments (available for sale), Gross Unrealized (Loss)	(71)	(178)
Total short-term investments (available for sale), Fair value	$ 209,391	$ 300,676