Financial Instruments - Summary of Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments (available for sale), Fair value	$ 300,676	$ 185,488
Short-term, Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments (available for sale), Cost	139	1,361
Total short-term investments (available for sale), Gross Unrealized Gain	1	3
Total short-term investments (available for sale), Fair value	140	1,364
Short-term, Non-governmental Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments (available for sale), Cost	288,578	165,423
Total short-term investments (available for sale), Gross Unrealized Gain	331	433
Total short-term investments (available for sale), Gross Unrealized (Loss)	(176)	(37)
Total short-term investments (available for sale), Fair value	288,733	165,819
Short-term, Treasury and Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments (available for sale), Cost	11,799	18,307
Total short-term investments (available for sale), Gross Unrealized Gain	6	7
Total short-term investments (available for sale), Gross Unrealized (Loss)	(2)	(9)
Total short-term investments (available for sale), Fair value	11,803	18,305
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments (available for sale), Cost	300,516	185,091
Total short-term investments (available for sale), Gross Unrealized Gain	338	443
Total short-term investments (available for sale), Gross Unrealized (Loss)	(178)	(46)
Total short-term investments (available for sale), Fair value	$ 300,676	$ 185,488