Employee Benefit Plans (Disaggregated Information Related to Corporations Defined Benefit Pension Plans) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 182,607	$ 176,751	$ 194,140	$ 199,637
Accumulated benefit obligation	182,607	176,751	194,140
Fair value of plan assets	168,450	157,214	163,270	$ 168,185
Funded status of the plan at end of year and accrued benefit (liability) recognized	(14,157)	(19,537)	$ (30,870)
Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	166,191	160,778
Accumulated benefit obligation	166,191	160,778
Fair value of plan assets	168,450	157,214
Funded status of the plan at end of year and accrued benefit (liability) recognized	2,259	(3,564)
Restoration Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	16,416	15,973
Accumulated benefit obligation	16,416	15,973
Fair value of plan assets	0	0
Funded status of the plan at end of year and accrued benefit (liability) recognized	$ (16,416)	$ (15,973)

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

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- Definition

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

+ References

+ Details

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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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No definition available.

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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.

+ 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 (c)
-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 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64852052&loc=d3e1703-114919

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