Accumulated Other Comprehensive Income - Components of Changes in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance		$ 296,608
Other comprehensive income		1,878
Income taxes	$ (366)	(798)	$ (900)
Ending Balance	323,350	323,350
Currency Translation Adjustments [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance		(563)
Other comprehensive income		(1,322)
Ending Balance	(1,885)	(1,885)
Debt Security [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance		4,350
Other comprehensive income		3,200
Income taxes		(798)
Ending Balance	6,752	6,752
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance		3,787
Ending Balance	$ 4,867	$ 4,867