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, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income Loss [Line Items]
Balance, Value	$ 86,780	$ 50,725	$ 105,044
Other comprehensive income (loss) before reclassifications	(49)	(68)	61
Amount reclassified from accumulated other comprehensive loss (income)	5	(5)	(23)
Net current period other comprehensive income (loss)	(44)	(73)	38
Balance, Value	133,630	86,780	50,725
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance, Value	(70)	3	(35)
Net current period other comprehensive income (loss)	(44)	(73)	38
Balance, Value	$ (114)	$ (70)	$ 3