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Changes in Accumulated Other Comprehensive Income (Loss) (Parenthetical) (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Amounts reclassified from accumulated other comprehensive income, tax $ (451) $ (445) $ (559)
Foreign currency translation adjustments, tax 0 0 0
Unrealized gains on marketable securities, tax 1,815 209 (459)
Retirement plans prior service benefit, tax (932) (778) (622)
Foreign currency translation reclassification adjustments      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Other comprehensive (loss) income before reclassifications, tax 0 0  
Amounts reclassified from accumulated other comprehensive income, tax 0 0  
Other comprehensive (loss) income, tax 0 0  
Unrealized Gains (Losses) on Marketable Securities      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Other comprehensive (loss) income before reclassifications, tax 1,903 923  
Amounts reclassified from accumulated other comprehensive income, tax (297) (255) (390)
Other comprehensive (loss) income, tax 1,606 668  
Amortization of prior service credit      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Other comprehensive (loss) income before reclassifications, tax 0 34  
Amounts reclassified from accumulated other comprehensive income, tax (154) (190) $ (169)
Other comprehensive (loss) income, tax $ (154) $ (156)