Available-for-Sale Marketable Securities in Unrealized Position (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 37,396	$ 40,121
Less than 12 Months, Unrealized Loss	(27)	(24)
12 Months or Longer, Fair Value	2,000
12 Months or Longer, Unrealized Loss	(1)
Total, Fair Value	39,396	40,121
Total, Unrealized Loss	(28)	(24)
United States government agency securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	11,771	6,035
Less than 12 Months, Unrealized Loss	(10)	(1)
Total, Fair Value	11,771	6,035
Total, Unrealized Loss	(10)	(1)
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	25,625	34,086
Less than 12 Months, Unrealized Loss	(17)	(23)
12 Months or Longer, Fair Value	2,000
12 Months or Longer, Unrealized Loss	(1)
Total, Fair Value	27,625	34,086
Total, Unrealized Loss	$ (18)	$ (23)