Changes in Accumulated Balances of Other Comprehensive Income (Loss), Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			$ 244,670
Other comprehensive income (loss) before reclassifications	$ (388)	$ 48	(1,004)	$ 359
Amounts reclassified from accumulated other comprehensive income (loss)	159	(37)	173	(140)
Other comprehensive income (loss), net of taxes	(229)	11	(831)	219
Ending balance	244,366		244,366
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(1,188)	(289)	(586)	(497)
Ending balance	(1,417)	(278)	(1,417)	(278)
Unrealized gains (losses) on available-for-sale marketable securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(1,184)	(327)	(586)	(502)
Other comprehensive income (loss) before reclassifications	(159)	48	(754)	195
Amounts reclassified from accumulated other comprehensive income (loss)	1	1	(2)	29
Other comprehensive income (loss), net of taxes	(158)	49	(756)	224
Ending balance	(1,342)	(278)	(1,342)	(278)
Unrealized gains (losses) on cash flow hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(4)	38		5
Other comprehensive income (loss) before reclassifications	(229)		(250)	164
Amounts reclassified from accumulated other comprehensive income (loss)	158	(38)	175	(169)
Other comprehensive income (loss), net of taxes	(71)	$ (38)	(75)	$ (5)
Ending balance	$ (75)		$ (75)