Changes in Accumulated Balances of Other Comprehensive Income (Loss), Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ 244,670
Other comprehensive income (loss) before reclassifications	(616)	$ 311
Amounts reclassified from accumulated other comprehensive income (loss)	14	(103)
Other comprehensive income (loss), net of taxes	(602)	208
Ending balance	252,815
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(586)	(497)
Ending balance	(1,188)	(289)
Unrealized gains (losses) on available-for-sale marketable securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(586)	(502)
Other comprehensive income (loss) before reclassifications	(595)	147
Amounts reclassified from accumulated other comprehensive income (loss)	(3)	28
Other comprehensive income (loss), net of taxes	(598)	175
Ending balance	(1,184)	(327)
Unrealized gains (losses) on cash flow hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	0	5
Other comprehensive income (loss) before reclassifications	(21)	164
Amounts reclassified from accumulated other comprehensive income (loss)	17	(131)
Other comprehensive income (loss), net of taxes	(4)	33
Ending balance	$ (4)	$ 38