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Pension Plans - Allocation of Defined Benefit Pension Plans Assets (Detail) - Pension Plan [Member] - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
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Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 69,245 | $ 65,446 | $ 65,439
Cash [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 642 | 728 |
Mutual Funds [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 68,603 | $ 64,718 |

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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 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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