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Schedule of changes in aggregate actuarial benefit obligation (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]      
Beginning balance $ 8,390 $ 7,631  
Plan change 69    
Service cost 184 180 $ 128
Interest cost 382 334 336
Actuarial loss 367 363  
Benefits paid (211) (118)  
Ending balance $ 9,181 $ 8,390 $ 7,631