Pension Plans - Net Periodic Pension Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Net settlement loss	$ 1,700	$ 1,700	$ 2,400
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	3,019	3,173	3,423
Expected return on plan assets	(4,202)	(4,131)	(4,627)
Recognized net actuarial loss	919	1,047	917
Net settlement loss	1,734	1,715	2,356
Total net periodic pension (income) expense	$ 1,470	$ 1,804	$ 2,069

Amount of gain (loss), recognized immediately as component of net periodic benefit (cost) credit, for change in value of benefit obligation or plan assets from experience different from that assumed, change in actuarial assumption, or consequence of temporarily deviating from substantive plan. Includes, but is not limited to, amount in excess of ten percent of greater of fair value of plan assets or benefit obligation.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 30
-Section 35
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=64937038&loc=d3e11661-114930

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of expected return (loss) recognized in net periodic benefit (cost) credit, calculated based on expected long-term rate of return and market-related value of plan assets of defined benefit plan.

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (h)(2)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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Amount of net periodic benefit cost (credit) for defined benefit plan.

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- Definition

Amount of gain (loss) recognized in net periodic benefit (cost) credit from irrevocable action relieving primary responsibility for benefit obligation and eliminating risk related to obligation and assets used to effect settlement.

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