Financial Instruments - Summary of Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Short-term, Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments (available for sale), Cost	$ 1,224	$ 1,361
Total short-term investments (available for sale), Gross Unrealized Gain	4	3
Total short-term investments (available for sale), Gross Unrealized (Loss)	(1)
Total short-term investments (available for sale), Fair value	1,227	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	206,184	165,423
Total short-term investments (available for sale), Gross Unrealized Gain	351	433
Total short-term investments (available for sale), Gross Unrealized (Loss)	(889)	(37)
Total short-term investments (available for sale), Fair value	205,646	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	36,687	18,307
Total short-term investments (available for sale), Gross Unrealized Gain	92	7
Total short-term investments (available for sale), Gross Unrealized (Loss)	(1)	(9)
Total short-term investments (available for sale), Fair value	36,778	18,305
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments (available for sale), Cost	244,095	185,091
Total short-term investments (available for sale), Gross Unrealized Gain	447	443
Total short-term investments (available for sale), Gross Unrealized (Loss)	(891)	(46)
Total short-term investments (available for sale), Fair value	$ 243,651	$ 185,488