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Retirement Plans (Details) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2018 | Mar. 31, 2017
Net Periodic Benefit Cost [Line Items] |  |
Employer matching contribution, Percentage | 5.00% |
Employees' Pension Plan |  |
Net Periodic Benefit Cost [Line Items] |  |
Service Cost 1 | $ 348 | $ 350
Interest Cost 2 | 525 | 350
Expected Return on Plan Assets 2 | (785) | (800)
Amortization of Prior Service (Credit) Cost 2 | (12) | (14)
Amortization of Net Loss 2 | 33 | 148
Net Periodic Benefit Cost | 109 | 34
Plan Contributions During the Period | 0 | 0
Select Executive Retirement Plan |  |
Net Periodic Benefit Cost [Line Items] |  |
Service Cost 1 | 11 | 10
Interest Cost 2 | 50 | 50
Expected Return on Plan Assets 2 | 0 | 0
Amortization of Prior Service (Credit) Cost 2 | 14 | 14
Amortization of Net Loss 2 | 33 | 30
Net Periodic Benefit Cost | 108 | 104
Plan Contributions During the Period | 116 | 113
Post-Retirement Benefit Plans |  |
Net Periodic Benefit Cost [Line Items] |  |
Service Cost 1 | 33 | 37
Interest Cost 2 | 99 | 86
Expected Return on Plan Assets 2 | 0 | 0
Amortization of Prior Service (Credit) Cost 2 | (3) | (3)
Amortization of Net Loss 2 | (6) | 0
Net Periodic Benefit Cost | 123 | 120
Plan Contributions During the Period | $ 17 | $ 53
On Or Subsequent to January 1, 2003 |  |
Net Periodic Benefit Cost [Line Items] |  |
Service credits, percent of eligible salaries | 6.00% |
Minimum | Prior to January 1, 2003 |  |
Net Periodic Benefit Cost [Line Items] |  |
Service credits, percent of eligible salaries | 6.00% |
Maximum | Prior to January 1, 2003 |  |
Net Periodic Benefit Cost [Line Items] |  |
Service credits, percent of eligible salaries | 12.00% |

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- Definition Amount of gain (loss) recognized in net periodic benefit (cost) credit 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 (h)(4) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of prior service cost (credit) recognized in net periodic benefit cost (credit) 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 (h)(5) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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