Financial Instruments - Summary of Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments (available for sale), Fair value	$ 185,488	$ 196,428
Short-term, Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments (available for sale), Cost	1,361
Total short-term investments (available for sale), Gross Unrealized Gain	3
Total short-term investments (available for sale), Fair value	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	165,423	179,393
Total short-term investments (available for sale), Gross Unrealized Gain	433	35
Total short-term investments (available for sale), Gross Unrealized (Loss)	(37)	(337)
Total short-term investments (available for sale), Fair value	165,819	179,091
Short-term, Treasury and Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments (available for sale), Cost	18,307	17,417
Total short-term investments (available for sale), Gross Unrealized Gain	7	5
Total short-term investments (available for sale), Gross Unrealized (Loss)	(9)	(85)
Total short-term investments (available for sale), Fair value	18,305	17,337
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments (available for sale), Cost	185,091	196,810
Total short-term investments (available for sale), Gross Unrealized Gain	443	40
Total short-term investments (available for sale), Gross Unrealized (Loss)	(46)	(422)
Total short-term investments (available for sale), Fair value	$ 185,488	$ 196,428