Available-for-Sale Marketable Securities in Unrealized Position (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 89,738	$ 44,514
Less than 12 Months, Unrealized Loss	(426)	(97)
Total, Fair Value	89,738	44,514
Total, Unrealized Loss	(426)	(97)
United States government agency securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	16,914	8,729
Less than 12 Months, Unrealized Loss	(39)	(24)
Total, Fair Value	16,914	8,729
Total, Unrealized Loss	(39)	(24)
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	72,824	35,785
Less than 12 Months, Unrealized Loss	(387)	(73)
Total, Fair Value	72,824	35,785
Total, Unrealized Loss	$ (387)	$ (73)