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Employee Benefits - Additional Information (Detail) - USD ($) | 12 Months Ended
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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 |  |

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