Available-for-Sale Marketable Securities in Unrealized Position (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 90,648	$ 44,514
Less than 12 Months, Unrealized Loss	(314)	(97)
12 Months or Longer, Fair Value	4,990
12 Months or Longer, Unrealized Loss	(12)
Total, Fair Value	95,638	44,514
Total, Unrealized Loss	(326)	(97)
United States government agency securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	17,908	8,729
Less than 12 Months, Unrealized Loss	(35)	(24)
12 Months or Longer, Fair Value	2,994
12 Months or Longer, Unrealized Loss	(6)
Total, Fair Value	20,902	8,729
Total, Unrealized Loss	(41)	(24)
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	72,740	35,785
Less than 12 Months, Unrealized Loss	(279)	(73)
12 Months or Longer, Fair Value	1,996
12 Months or Longer, Unrealized Loss	(6)
Total, Fair Value	74,736	35,785
Total, Unrealized Loss	$ (285)	$ (73)