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Post Retirement Benefits (Details) | 12 Months Ended
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Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) Plan | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2010 USD ($)
Postretirement benefits: |  |  |  |  |
Number of 401k plans | Plan |  | 3 |  |  |
Amounts that will be amortized from accumulated other comprehensive income in next fiscal year: |  |  |  |  |
Interest cost |  | $ 323,000 | $ 316,000 | $ 277,000 |
Postretirement benefits |  |  |  |  |
Postretirement benefits: |  |  |  |  |
Decrease in post retirement obligations |  |  |  |  | $ 10,282,000
Amortization period |  |  |  |  | 20 years
Change in net periodic benefit cost due to plan amendment |  | 496,000 | 496,000 | $ 496,000 |
Defined Benefit Plan, Actuarial Gain (Loss) |  | 320,000 | (48,000) |  |
Prior service credit not yet recognized |  | 3,634,000 | 3,655,000 |  |
Amounts that will be amortized from accumulated other comprehensive income in next fiscal year: |  |  |  |  |
Interest cost |  | $ 323,000 | $ 316,000 |  |
Assumed health care cost trend rates: |  |  |  |  |
Health care cost trend rate assumed for next fiscal year |  | 7.00% | 6.00% |  |
Ultimate health care cost trend rate |  | 5.00% | 5.00% |  |
Effect of changing the health care cost trend rate by one-percentage point for each future year: |  |  |  |  |
Effect on total of service and interest cost components, 1 - percentage point increase |  | $ 43,000 |  |  |
Effect on total of service and interest cost components, 1 - percentage point decrease |  | (50,000) |  |  |
Effect on post retirement benefit obligation, 1 - percentage point increase |  | 941,000 |  |  |
Effect on post retirement benefit obligation, 1 - percentage point decrease |  | $ (800,000) |  |  |
Forecast | Postretirement benefits |  |  |  |  |
Postretirement benefits: |  |  |  |  |
Change in net periodic benefit cost due to plan amendment | $ 496,000 |  |  |  |
Defined Benefit Plan, Actuarial Gain (Loss) | 49,000 |  |  |  |
Amounts that will be amortized from accumulated other comprehensive income in next fiscal year: |  |  |  |  |
Amortization of prior service credit, net of tax | (496,000) |  |  |  |
Amortization of loss, net of tax | (149,000) |  |  |  |
Interest cost | $ 298,000 |  |  |  |

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- Definition Amount of gain (loss) related to change in benefit obligation resulting from changes in actuarial assumptions, for example, but not limited to, interest, mortality, employee turnover, salary, and temporary deviation from the substantive plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Gain or Loss -URI http://asc.fasb.org/extlink&oid=49168827 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)(4) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition Amount of effect of a one-percentage-point decrease in the assumed health care cost trend rates on the accumulated postretirement benefit obligation for health care benefits. + 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 (m) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition The assumed health care cost trend rate for the next year used to measure the expected cost of benefits covered by the plan (gross eligible charges). This is based upon the annual rate of change in the cost of health care benefits currently provided by the postretirement benefit plan, due to factors other than changes in the composition of the plan population by age and dependency status. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Health Care Cost Trend Rate -URI http://asc.fasb.org/extlink&oid=6514820 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (l) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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