Available-for-Sale Marketable Securities in Unrealized Position (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value		$ 75,761
Less than 12 Months, Unrealized Loss		(253)
12 Months or Longer, Fair Value	$ 54,956	10,975
12 Months or Longer, Unrealized Loss	(88)	(30)
Total, Fair Value	54,956	86,736
Total, Unrealized Loss	(88)	(283)
United States government agency securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value		14,948
Less than 12 Months, Unrealized Loss		(22)
12 Months or Longer, Fair Value	7,477	999
12 Months or Longer, Unrealized Loss	(11)	(1)
Total, Fair Value	7,477	15,947
Total, Unrealized Loss	(11)	(23)
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value		60,813
Less than 12 Months, Unrealized Loss		(231)
12 Months or Longer, Fair Value	47,479	9,976
12 Months or Longer, Unrealized Loss	(77)	(29)
Total, Fair Value	47,479	70,789
Total, Unrealized Loss	$ (77)	$ (260)