Financial Instruments - Summary of Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Short-term, Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments (available for sale), Cost	$ 628
Total short-term investments (available for sale), Gross Unrealized Gain	1
Total short-term investments (available for sale), Fair value	629
Short-term, Non-governmental Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments (available for sale), Cost	212,175	$ 179,393
Total short-term investments (available for sale), Gross Unrealized Gain	420	35
Total short-term investments (available for sale), Gross Unrealized (Loss)	(28)	(337)
Total short-term investments (available for sale), Fair value	212,567	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	15,606	17,417
Total short-term investments (available for sale), Gross Unrealized Gain	6	5
Total short-term investments (available for sale), Gross Unrealized (Loss)	(13)	(85)
Total short-term investments (available for sale), Fair value	15,599	17,337
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments (available for sale), Cost	228,409	196,810
Total short-term investments (available for sale), Gross Unrealized Gain	427	40
Total short-term investments (available for sale), Gross Unrealized (Loss)	(41)	(422)
Total short-term investments (available for sale), Fair value	$ 228,795	$ 196,428