Financial Instruments - Summary of Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Short-term, Non-governmental Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments (available for sale), Cost	$ 187,935	$ 179,393
Total short-term investments (available for sale), Gross Unrealized Gain	150	35
Total short-term investments (available for sale), Gross Unrealized (Loss)	(95)	(337)
Total short-term investments (available for sale), Fair value	187,990	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	12,816	17,417
Total short-term investments (available for sale), Gross Unrealized Gain	4	5
Total short-term investments (available for sale), Gross Unrealized (Loss)	(42)	(85)
Total short-term investments (available for sale), Fair value	12,778	17,337
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments (available for sale), Cost	200,751	196,810
Total short-term investments (available for sale), Gross Unrealized Gain	154	40
Total short-term investments (available for sale), Gross Unrealized (Loss)	(137)	(422)
Total short-term investments (available for sale), Fair value	$ 200,768	$ 196,428