Changes in Accumulated Balances of Other Comprehensive Income (Loss), Net of Tax (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ 211,551		$ 186,095	$ 179,049
Other comprehensive income (loss) before reclassifications	80		57
Amounts reclassified from accumulated other comprehensive income (loss)	(169)		(135)
Other comprehensive income (loss), net of taxes	(89)		(78)	17
Ending balance	244,670		211,551	186,095
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(497)		(419)	(436)
Other comprehensive income (loss), net of taxes	(89)		(78)	17
Ending balance	(586)	[1]	(497)	(419)
Unrealized gains (losses) on available-for-sale marketable securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(502)		(427)
Other comprehensive income (loss) before reclassifications	(83)		(83)
Amounts reclassified from accumulated other comprehensive income (loss)	(1)		8
Other comprehensive income (loss), net of taxes	(84)		(75)
Ending balance	(586)		(502)	(427)
Unrealized gains on cash flow hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	5		8
Other comprehensive income (loss) before reclassifications	163		140
Amounts reclassified from accumulated other comprehensive income (loss)	(168)		(143)
Other comprehensive income (loss), net of taxes	(5)		(3)
Ending balance	$ 0		$ 5	$ 8

[1] Accumulated other comprehensive loss for the year ended December 31, 2017 is all from available-for-sale securities, net of taxes of $92.