Financial Instruments - Summary of Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 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	$ 139	$ 139
Total short-term investments (available for sale), Gross Unrealized Gain	1	1
Total short-term investments (available for sale), Fair value	140	140
Short-term, Non-governmental Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments (available for sale), Cost	354,136	288,578
Total short-term investments (available for sale), Gross Unrealized Gain	145	331
Total short-term investments (available for sale), Gross Unrealized (Loss)	(208)	(176)
Total short-term investments (available for sale), Fair value	354,073	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	6,770	11,799
Total short-term investments (available for sale), Gross Unrealized Gain	5	6
Total short-term investments (available for sale), Gross Unrealized (Loss)	(2)	(2)
Total short-term investments (available for sale), Fair value	6,773	11,803
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments (available for sale), Cost	361,045	300,516
Total short-term investments (available for sale), Gross Unrealized Gain	151	338
Total short-term investments (available for sale), Gross Unrealized (Loss)	(210)	(178)
Total short-term investments (available for sale), Fair value	$ 360,986	$ 300,676