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Pension Plans and Other Postretirement Benefits - Components of Net Periodic Cost, Changes in Plan Assets and Benefit Obligations, and AOCI Impacts (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Amortization of: |  |  |
Total recognized in other comprehensive income (loss) | $ (1,461) | $ 37 | $ (1,815)
Defined Benefit Plan |  |  |
Service cost: |  |  |
Benefit accrual | 0 | 0 | 0
Other expenses | 650 | 650 | 450
Interest cost | 1,091 | 1,244 | 1,189
Expected return on plan assets | (1,493) | (1,675) | (1,875)
Settlement loss | 0 | 0 | 0
Amortization of: |  |  |
Prior service cost | 0 | 0 | 0
Actuarial loss | 389 | 393 | 1,626
Net periodic pension expense | 637 | 612 | 1,390
Changes in plan assets and benefit obligations included in other comprehensive income (loss): |  |  |
Prior service cost | 0 | 0 | 0
Net actuarial loss (gain) | (1,619) | 175 | 930
Amortization of: |  |  |
Prior service cost | 0 | 0 | 0
Actuarial loss | (389) | (393) | (1,626)
Total recognized in other comprehensive income (loss) | $ (2,008) | $ (218) | $ (696)
Weighted average assumptions used to determine expense: |  |  |
Discount rate | 3.90% | 4.20% | 3.66%
Expected return on plan assets | 6.25% | 6.50% | 6.75%
Weighted average assumptions used to determine benefit obligations as of December 31: |  |  |
Discount rate | 3.50% | 3.90% | 4.20%
Expected return on plan assets | 6.25% | 6.50% | 6.75%
Supplemental Defined Benefit Plans |  |  |
Service cost: |  |  |
Benefit accrual | $ 0 | $ 0 | $ 0
Other expenses | 0 | 0 | 0
Interest cost | 631 | 687 | 654
Expected return on plan assets | 0 | 0 | 0
Settlement loss | 0 | 0 | 0
Amortization of: |  |  |
Prior service cost | 0 | 0 | 0
Actuarial loss | 258 | 233 | 273
Net periodic pension expense | 889 | 920 | 927
Changes in plan assets and benefit obligations included in other comprehensive income (loss): |  |  |
Prior service cost | 0 | 0 | 0
Net actuarial loss (gain) | 805 | 488 | (845)
Amortization of: |  |  |
Prior service cost | 0 | 0 | 0
Actuarial loss | (258) | (233) | (273)
Total recognized in other comprehensive income (loss) | $ 547 | $ 255 | $ (1,118)
Weighted average assumptions used to determine expense: |  |  |
Discount rate | 3.90% | 4.20% | 3.66%
Weighted average assumptions used to determine benefit obligations as of December 31: |  |  |
Discount rate | 3.50% | 3.90% | 4.20%

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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 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 Amount, before tax, of reclassification adjustment from accumulated other comprehensive (income) loss for prior service 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 (i) -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 220 -SubTopic 10 -Section 45 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e689-108580 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e637-108580 + Details
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