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	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income Loss [Line Items]
Balance, Value	$ 86,780	$ 50,725
Other comprehensive income (loss) before reclassifications	(59)	24
Amount reclassified from accumulated other comprehensive loss (income)	15	(3)
Net current period other comprehensive income (loss)	(44)	21
Balance, Value	72,103	29,269
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance, Value	(70)	3
Net current period other comprehensive income (loss)	(44)	21
Balance, Value	$ (114)	$ 24