Amounts Recognized in Accumulated Other Comprehensive Loss Net of Tax (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	$ 490	$ 170
Prior service cost	705	712
Gross amount recognized	1,195	882
Deferred income taxes	(276)	(195)
Net amount recognized	$ 919	$ 687