Retirement Plans - Information about Pension Plan (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in benefit obligation:
Beginning benefit obligation	$ 16,964	$ 16,328
Service cost	0	0	$ 0
Interest cost	679	689	604
Curtailment gain	0	0
Settlement loss	46	51
Actuarial (gain)/loss	986	669
Benefits paid	(759)	(773)
Ending benefit obligation	17,916	16,964	16,328
Change in plan assets, at fair value:
Beginning plan assets	16,150	15,647
Actual return	1,947	802
Employer contribution	2,000	500
Benefits paid	(759)	(773)
Administrative expenses	(32)	(26)
Ending plan assets	19,306	16,150	$ 15,647
Funded status at end of year	$ 1,390	$ (814)

Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses.

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Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan.

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Amount of administration expense of defined benefit plan which decreases plan assets. Excludes plan administration expense paid by employer.

+ References

No definition available.

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Amount of actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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Amount of payment to participant of defined benefit plan which decreases benefit obligation. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 60
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=6414203&loc=d3e39716-114964

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-Publisher FASB
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-Section 50
-Paragraph 1
-Subparagraph (a)(6)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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No definition available.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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No definition available.

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- Definition

Amount of contribution received by defined benefit plan from employer which increases plan assets.

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-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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- Definition

Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

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- Definition

Amount of funded (unfunded) status of defined benefit plan, measured as difference between fair value of plan assets and benefit obligation. Includes, but is not limited to, overfunded (underfunded) status.

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- Definition

Amount of cost recognized for passage of time related to defined benefit plan.

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- Definition

Amount of payment to participant under defined benefit plan which decreases plan assets. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 60
-Section 50
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- Definition

Amount of gain (loss) recognized in net periodic benefit (cost) credit from event reducing expected years of future service of present employees or eliminating accrual of defined benefits for some or all future services of present employees.

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Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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- Definition

Amount of (increase) decrease to benefit obligation of defined benefit plan from irrevocable action relieving primary responsibility for benefit obligation and eliminating risk for obligation and assets used to effect settlement. Includes, but is not limited to, lump-sum cash payment to participant in exchange for right to receive specified benefits, purchase of nonparticipating annuity contract and change from remeasurement.

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