Accumulated Other Comprehensive Income (Loss) - Summary of Changes in Accumulated Other Comprehensive Income (Loss), Net of Tax, from Unrealized Gains (Losses) on Available-for-Sale Marketable Securities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income Loss [Line Items]
Balance, Value	$ 50,725	$ 105,044	$ 51,662
Other comprehensive income (loss) before reclassifications	(68)	61	(35)
Amount reclassified from accumulated other comprehensive income	(5)	(23)
Net current period other comprehensive income (loss)	(73)	38	(35)
Balance, Value	86,780	50,725	105,044
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance, Value	3	(35)
Net current period other comprehensive income (loss)	(73)	38	(35)
Balance, Value	$ (70)	$ 3	$ (35)