Retirement Plans - Non-contributory defined benefit pension plan - (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Change in benefit obligation:
Interest cost	$ 281	$ 283	$ 562	$ 566
Components of net periodic pension cost and other amounts recognized in other comprehensive income
Interest cost	(281)	(283)	(562)	(566)
Expected return on plan assets	553	534	1,107	1,068
Recognized net actuarial loss	(188)	(204)	(376)	(408)
Net periodic pension benefit	$ 84	$ 47	$ 169	$ 94
Employee hired on or after January 1, 2006 \| Maximum
Retirement Plans
Requisite age of employees for receiving retirement benefits under the new benefit formula			45 years
Defined benefit plan requisite service period under new benefit formula			5 years
Non-contributory defined benefit pension plan \| Employees hired on or before December 31, 2005 \| Minimum
Retirement Plans
Requisite age of employees for receiving retirement benefits under the plan			21 years

Represents the age of employees to be eligible to participate in the defined benefit plan.

No definition available.

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- Definition

Represents the age of employees to be eligible to participate in the defined benefit plan under new benefit formula.

+ References

No definition available.

+ Details

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- Definition

Represents the defined benefit plan recognized net actuarial loss.

+ References

No definition available.

+ Details

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- Definition

Defined Benefit Plan Requisite Service Period Under New Benefit Formula

+ References

No definition available.

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- References

No definition available.

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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.

+ References

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

An amount calculated as a basis for determining the extent of delayed recognition of the effects of changes in the fair value of assets. The expected return on plan assets is determined based on the expected long-term rate of return on plan assets and the market-related value of plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Expected Return on Plan Assets
-URI http://asc.fasb.org/extlink&oid=6512171

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (h)(3)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920

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- Definition

The increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time.

+ 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=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 1
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920

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- Definition

The total amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments.

+ 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)
-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 1
-Subparagraph (j)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920

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