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Financial Instruments - Summary of Investments (Detail) - USD ($) $ in Thousands	Sep. 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	$ 846	$ 1,361
Total short-term investments (available for sale), Gross Unrealized Gain	3	3
Total short-term investments (available for sale), Fair value	849	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	287,248	165,423
Total short-term investments (available for sale), Gross Unrealized Gain	636	433
Total short-term investments (available for sale), Gross Unrealized (Loss)	(129)	(37)
Total short-term investments (available for sale), Fair value	287,755	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	16,989	18,307
Total short-term investments (available for sale), Gross Unrealized Gain	19	7
Total short-term investments (available for sale), Gross Unrealized (Loss)	(2)	(9)
Total short-term investments (available for sale), Fair value	17,006	18,305
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments (available for sale), Cost	305,083	185,091
Total short-term investments (available for sale), Gross Unrealized Gain	658	443
Total short-term investments (available for sale), Gross Unrealized (Loss)	(131)	(46)
Total short-term investments (available for sale), Fair value	$ 305,610	$ 185,488

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