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Pension and Other Postretirement Benefits - Fair Values of Pension Plan Assets by Asset Category (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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Pension And Other Employee Benefit Plans [Line Items] |  |
Fair values of pension plan assets by asset category | $ 198,156 | $ 182,370
Large-Cap Fund [Member] |  |
Pension And Other Employee Benefit Plans [Line Items] |  |
Fair values of pension plan assets by asset category | 75,165 | 65,495
Mid-Cap Fund [Member] |  |
Pension And Other Employee Benefit Plans [Line Items] |  |
Fair values of pension plan assets by asset category | 13,046 | 11,419
Small-Cap Fund [Member] |  |
Pension And Other Employee Benefit Plans [Line Items] |  |
Fair values of pension plan assets by asset category | 18,785 | 17,184
Government Bond Fund [Member] |  |
Pension And Other Employee Benefit Plans [Line Items] |  |
Fair values of pension plan assets by asset category | 28,429 | 26,151
Corporate Bond Fund [Member] |  |
Pension And Other Employee Benefit Plans [Line Items] |  |
Fair values of pension plan assets by asset category | 24,421 | 20,971
Fixed Income Fund [Member] |  |
Pension And Other Employee Benefit Plans [Line Items] |  |
Fair values of pension plan assets by asset category | 38,072 | 40,958
Cash & Equivalents [Member] |  |
Pension And Other Employee Benefit Plans [Line Items] |  |
Fair values of pension plan assets by asset category | 238 | 192
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] |  |
Pension And Other Employee Benefit Plans [Line Items] |  |
Fair values of pension plan assets by asset category | 238 | 192
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Large-Cap Fund [Member] |  |
Pension And Other Employee Benefit Plans [Line Items] |  |
Fair values of pension plan assets by asset category |  | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Mid-Cap Fund [Member] |  |
Pension And Other Employee Benefit Plans [Line Items] |  |
Fair values of pension plan assets by asset category |  | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Small-Cap Fund [Member] |  |
Pension And Other Employee Benefit Plans [Line Items] |  |
Fair values of pension plan assets by asset category |  | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Government Bond Fund [Member] |  |
Pension And Other Employee Benefit Plans [Line Items] |  |
Fair values of pension plan assets by asset category |  | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Corporate Bond Fund [Member] |  |
Pension And Other Employee Benefit Plans [Line Items] |  |
Fair values of pension plan assets by asset category |  | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Cash & Equivalents [Member] |  |
Pension And Other Employee Benefit Plans [Line Items] |  |
Fair values of pension plan assets by asset category | 238 | 192
Significant Observable Inputs (Level 2) [Member] |  |
Pension And Other Employee Benefit Plans [Line Items] |  |
Fair values of pension plan assets by asset category | 38,072 | 40,958
Significant Observable Inputs (Level 2) [Member] | Fixed Income Fund [Member] |  |
Pension And Other Employee Benefit Plans [Line Items] |  |
Fair values of pension plan assets by asset category | $ 38,072 | $ 40,958

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- Definition Pension And Other Employee Benefit Plans [Line Items] + References No definition available. + Details
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- Definition Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees. + 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 (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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