Available-for-Sale Marketable Securities in Unrealized Position (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 9,577	$ 75,761
Less than 12 Months, Unrealized Loss	(7)	(253)
12 Months or Longer, Fair Value	25,499	10,975
12 Months or Longer, Unrealized Loss	(10)	(30)
Total, Fair Value	35,076	86,736
Total, Unrealized Loss	(17)	(283)
United States government agency securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	5,527	14,948
Less than 12 Months, Unrealized Loss	(2)	(22)
12 Months or Longer, Fair Value	3,496	999
12 Months or Longer, Unrealized Loss	(1)	(1)
Total, Fair Value	9,023	15,947
Total, Unrealized Loss	(3)	(23)
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	4,050	60,813
Less than 12 Months, Unrealized Loss	(5)	(231)
12 Months or Longer, Fair Value	22,003	9,976
12 Months or Longer, Unrealized Loss	(9)	(29)
Total, Fair Value	26,053	70,789
Total, Unrealized Loss	$ (14)	$ (260)