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Financial Instruments - Summary of Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 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	$ 1,538	$ 1,361
Total short-term investments (available for sale), Gross Unrealized Gain	2	3
Total short-term investments (available for sale), Fair value	1,540	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	219,215	165,423
Total short-term investments (available for sale), Gross Unrealized Gain	975	433
Total short-term investments (available for sale), Gross Unrealized (Loss)	(37)	(37)
Total short-term investments (available for sale), Fair value	220,153	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	28,525	18,307
Total short-term investments (available for sale), Gross Unrealized Gain	49	7
Total short-term investments (available for sale), Gross Unrealized (Loss)	(2)	(9)
Total short-term investments (available for sale), Fair value	28,572	18,305
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments (available for sale), Cost	249,278	185,091
Total short-term investments (available for sale), Gross Unrealized Gain	1,026	443
Total short-term investments (available for sale), Gross Unrealized (Loss)	(39)	(46)
Total short-term investments (available for sale), Fair value	$ 250,265	$ 185,488

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