Increase (decrease) defined benefit plan net periodic benefit cost.

No definition available.

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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.

+ References

No definition available.

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- Definition

Amount of benefits from a defined benefit plan expected to be paid in the five fiscal years after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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- Definition

Amount of benefits from a defined benefit plan expected to be paid in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

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- Definition

Amount of benefits from a defined benefit plan expected to be paid in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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- Definition

Amount of benefits from a defined benefit plan expected to be paid in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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- Definition

Amount of benefits from a defined benefit plan expected to be paid in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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- Definition

Amount of benefits from a defined benefit plan expected to be paid in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

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- Definition

Target allocation maximum percentage of investments of this type to total plan assets presented on a weighted average basis.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (c)(i)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e2446-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(i)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920

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- Definition

Target allocation minimum percentage of investments of this type to total plan assets presented on a weighted average basis.

+ 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 (d)(i)
-URI http://asc.fasb.org/extlink&oid=65897934&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 5
-Subparagraph (c)(i)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e2446-114920

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- Definition

Transfers in and out of Level 3. For example, transfers due to changes in the observability of significant inputs.

+ 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 (d)(iv)(02)(C)
-URI http://asc.fasb.org/extlink&oid=65897934&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 5
-Subparagraph (c)(iv)(02)(C)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e2446-114920

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- Definition

Amount of discretionary contributions made by an employer to a defined contribution plan.

+ References

No definition available.

+ Details

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- Definition

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

+ References

No definition available.

+ Details

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- Definition

Maximum amount the employee may contribute to a defined contribution plan.

+ References

No definition available.

+ Details

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- Definition

Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.

+ References

No definition available.

+ Details

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Employee Benefits - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Benefits expected to be paid in 2017		$ 1,457,000
Benefits expected to be paid in 2018		1,481,000
Benefits expected to be paid in 2019		1,516,000
Benefits expected to be paid in 2020		1,671,000
Benefits expected to be paid in 2021		1,863,000
Aggregate benefits expected to paid		10,650,000
Discretionary bonus expense		1,418,000	$ 1,178,000	$ 1,434,000
Transfers in or out of level 3		$ 0	0
Voluntary contribution of employees		33.30%
Contributions matched by compensation contribution		6.00%
Voluntary contribution of employees, amount		$ 418,000	$ 403,000	$ 346,000
Change in Assumptions for Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Decrease in periodic plan cost		859,000
Supplemental Insurance/ Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefits expected to be paid in 2017		2,031,000
Benefits expected to be paid in 2018		1,990,000
Benefits expected to be paid in 2019		1,964,000
Benefits expected to be paid in 2020		1,906,000
Benefits expected to be paid in 2021		1,830,000
Aggregate benefits expected to paid		$ 11,972,000
Scenario, Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Discretionary bonus expense	$ 1,000,000
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Allocation mix for the common and collective trust portfolio, Minimum		40.00%
Allocation mix for the common and collective trust portfolio, Maximum		64.00%
Fixed Income Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Allocation mix for the common and collective trust portfolio, Minimum		15.00%
Allocation mix for the common and collective trust portfolio, Maximum		25.00%
Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Allocation mix for the common and collective trust portfolio, Minimum		20.00%
Allocation mix for the common and collective trust portfolio, Maximum		36.00%