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Schedule of Amounts Recognized in Accumulated Other Comprehensive Income (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plan Disclosure      
Deferred taxes $ (12,881) $ (9,398) $ (10,235)
United States Pension Plans, Defined Benefit      
Defined Benefit Plan Disclosure      
Prior service cost, before tax 58 127  
Deferred taxes 539 514  
Total recognized in other comprehensive income, net of tax 597 641  
Postretirement Benefit Plans      
Defined Benefit Plan Disclosure      
Prior service cost, before tax (622) (1,118)  
Deferred taxes (1,472) (1,292)  
Total recognized in other comprehensive income, net of tax $ (2,094) $ (2,410)