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Pension Plans and Other Postretirement Benefits - Fair Value of Plan Assets (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |
Total | $ 25,843 | $ 25,446
Fixed income funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total | 12,165 | 12,402
Short-term investment funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total | 588 | 716
Level 1 |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total | 588 | 716
Level 1 | Fixed income funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total | 0 | 0
Level 1 | Short-term investment funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total | 588 | 716
Level 2 |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total | 25,255 | 24,730
Level 2 | Fixed income funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total | 12,165 | 12,402
Level 2 | Short-term investment funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total | 0 | 0
Level 3 |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total | 0 | 0
Level 3 | Fixed income funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total | 0 | 0
Level 3 | Short-term investment funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total | 0 | 0
United States | Equity security funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total | 10,517 | 9,836
United States | Level 1 | Equity security funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total | 0 | 0
United States | Level 2 | Equity security funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total | 10,517 | 9,836
United States | Level 3 | Equity security funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total | 0 | 0
International | Equity security funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total | 2,573 | 2,492
International | Level 1 | Equity security funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total | 0 | 0
International | Level 2 | Equity security funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total | 2,573 | 2,492
International | Level 3 | Equity security funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total | $ 0 | $ 0

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Fair value of plan assets for defined benefit plan with benefit obligation in excess of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2417-114920 + Details
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