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Employee Benefits - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]      
Percentage of company stock held equal to plan assets 11.00%    
Period of obligations for retired lives in trust 30 years    
Long term inflation rate 2.50%    
Minimum      
Defined Benefit Plan Disclosure [Line Items]      
Expected rate of return on equity security 6.00%    
Expected rate of return on fixed income security 3.00%    
Overall expected rate of return 5.00%    
Maximum      
Defined Benefit Plan Disclosure [Line Items]      
Expected rate of return on equity security 9.00%    
Expected rate of return on fixed income security 5.00%    
Overall expected rate of return 7.00%    
Cash Equivalents      
Defined Benefit Plan Disclosure [Line Items]      
Weighted average asset allocation 2.00%    
Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Estimated unrecognized net actuarial loss (gain) that will be amortized from accumulated other comprehensive loss into net periodic benefit cost $ 7,200,000    
Amount recognized of net actuarial loss into net periodic benefit cost (8,209,000) $ (9,050,000) $ (8,208,000)
Estimated prior service cost that will be amortized from accumulated other comprehensive loss (gain) into net periodic benefit cost 0    
Amortization of past-service liability $ 0    
Discount rates used to determine the benefit obligation 3.40% 3.90%  
Percentage of assets allocated to equities to be considered well funded 55.00%    
Percentage of assets allocated to fixed income to be considered well funded 44.00%    
Post-Retirement Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Estimated unrecognized net actuarial loss (gain) that will be amortized from accumulated other comprehensive loss into net periodic benefit cost $ 309,000    
Amount recognized of net actuarial loss into net periodic benefit cost (274,000) $ (326,000) (383,000)
Estimated prior service cost that will be amortized from accumulated other comprehensive loss (gain) into net periodic benefit cost 249,000    
Amortization of past-service liability $ (249,000) $ (249,000) $ (249,000)
Discount rates used to determine the benefit obligation 3.30% 3.70%  
Effect of 1% increase in assumed medical trend rate on accumulated post-retirement benefit obligation $ 736,000    
Effect of 1% increase in assumed medical trend rate on benefits earned and the interest components 28,000    
Effect of 1% decrease in assumed medical trend rate on accumulated post-retirement benefit obligation 623,000    
Effect of 1% decrease in assumed medical trend rate on benefits earned and the interest components 24,000    
Contribution to Health & Welfare Plan to pay premiums and claims in the next fiscal year $ 1,300,000