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Retirement Plans - Additional Information (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items] |  |  |
Average service period | 5 years |  |
Comprehensive loss position | $ (404) | $ 371 | $ 1,301
Pension plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Estimated contribution to the pension plan | $ 165 |  |
Current investment policy targets: |  |  |
Equity and debt investment fluctuations by plus or minus | 20.00% |  |
Assumptions used in determining periodic pension cost: |  |  |
Rate of increase in periodic pension costs | 3.65% | 4.30% |
Expected long-term rate of return on plan assets | 7.50% | 7.50% |
Pension plan | Equities |  |  |
Current investment policy targets: |  |  |
Current investment policy target | 65.00% |  |
Current investment mix | 57.00% | 60.00% |
Pension plan | Equities | Maximum |  |  |
Current investment policy targets: |  |  |
Current investment policy target | 78.00% |  |
Pension plan | Equities | Minimum |  |  |
Current investment policy targets: |  |  |
Current investment policy target | 52.00% |  |
Pension plan | Debt |  |  |
Current investment policy targets: |  |  |
Current investment policy target | 25.00% |  |
Current investment mix | 37.00% | 29.00% |
Pension plan | Money market funds |  |  |
Current investment policy targets: |  |  |
Current investment policy target | 10.00% |  |
Current investment mix | 6.00% | 11.00% |
SERP |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Comprehensive loss position | $ (328) | $ (214) | $ (234)
Assumptions used in determining actuarial present value of projected benefits obligation: |  |  |
Weighted-average discount rate | 3.40% | 3.90% | 4.15%
Rate of increase in future compensation levels | 0.00% | 3.50% | 3.50%

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- Definition Defined Benefit Plan, Average Service Period + References No definition available. + Details
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- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of 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 (k)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Weighted average rate increase of compensation, used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, pay-related 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 (k)(2) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of 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 (k)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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