Benefit Plans - Change in Projected Benefit Obligation (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Accrued interest receivable	$ 993	$ 606
Pension Plan
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	161,306	157,661
Interest cost	5,713	6,681	$ 6,889
Actuarial loss	10,148	2,047
Benefits paid	(6,601)	(5,546)
Projected benefit obligation at end of year	170,566	161,306	157,661
Change in fair value of plan assets:
Fair value of plan assets at beginning of year	206,639	189,414
Actual return on plan assets	21,468	22,424
Employer contributions	618	347
Benefits paid	(6,601)	(5,546)
Fair value of plan assets at end of year	222,124	206,639	$ 189,414
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Asset recognized	51,558	45,333
Accumulated benefit obligation	$ 170,566	$ 161,306

Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level.

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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 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
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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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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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No definition available.

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- Definition

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

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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

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- Definition

Carrying amount as of the balance sheet date of interest earned but not received. Also called accrued interest or accrued interest receivable.

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- Definition

Amount of liability, recognized in statement of financial position, for defined benefit pension and other postretirement plans.

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