Retirement Plans - Non-contributory defined benefit pension plan - (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Components of net periodic pension cost and other amounts recognized in other comprehensive income
Interest cost	$ 270	$ 281	$ 540	$ 562
Service cost	19	31	39	63
Expected return on plan assets	(582)	(553)	(1,164)	(1,107)
Recognized net actuarial loss	194	188	387	376
Net periodic pension expense	$ (99)	$ (53)	$ (198)	$ (106)
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
Requisite age of employees for receiving retirement benefits under the 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.

+ Details

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- 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.

+ Details

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- Definition

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.

+ References

+ Details

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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

+ Details

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- Definition

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

+ References

+ Details

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- Definition

Amount of cost for actuarial present value of benefits attributed to service rendered by employee for 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)(1)
-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)(1)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

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