Financial Instruments - Summary of Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 103,148	$ 61,985
Gross Unrealized Gains	0	7
Gross Unrealized Losses	(303)	(156)
Estimated Fair Values	102,845	61,836
Corporate bonds and notes
Schedule of Available-for-sale Securities [Line Items]
Cost	45,803	36,590
Gross Unrealized Gains	0	7
Gross Unrealized Losses	(230)	(95)
Estimated Fair Values	45,573	36,502
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Cost	2,392	5,220
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2)	(4)
Estimated Fair Values	2,390	5,216
Treasury and agency notes and bills
Schedule of Available-for-sale Securities [Line Items]
Cost	6,000	6,029
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(71)	(57)
Estimated Fair Values	5,929	5,972
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Cost	8,540
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Estimated Fair Values	8,540
Money market funds
Schedule of Available-for-sale Securities [Line Items]
Cost	40,413	14,146
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Values	40,413	14,146
Cash and cash equivalents
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Values	40,413	14,457
Short-term investments
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Values	$ 62,432	$ 47,379