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Employee Benefit Plans (Pension Benefit Plan) - Schedule of Cash Equivalents and Investments Held at Fair Value (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |
Available-for-sale securities | $ 118,038 | $ 190,723 |
Total | 26,624 | 20,045 | $ 19,213
Corporate Bonds [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Available-for-sale securities | 32,543 | 66,412 |
Pension Benefit Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Cash and cash equivalents | 3,005 | 6 |
Available-for-sale securities | 23,619 | 20,039 |
Total | 26,624 | 20,045 |
Pension Benefit Plan [Member] | Corporate Bonds [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Available-for-sale securities | 14,349 | 12,546 |
Pension Benefit Plan [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Cash and cash equivalents | 3,005 | 6 |
Available-for-sale securities | 23,619 | 20,039 |
Total | 26,624 | 20,045 |
Pension Benefit Plan [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Corporate Bonds [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Available-for-sale securities | 14,349 | 12,546 |
Pension Benefit Plan [Member] | Government Bonds [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Available-for-sale securities | 2,305 | 2,037 |
Pension Benefit Plan [Member] | Government Bonds [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Available-for-sale securities | 2,305 | 2,037 |
Pension Benefit Plan [Member] | Large Cap Blend [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Available-for-sale securities | 5,758 | 4,462 |
Pension Benefit Plan [Member] | Large Cap Blend [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Available-for-sale securities | 5,758 | 4,462 |
Pension Benefit Plan [Member] | Large Cap Value [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Available-for-sale securities | 309 | 249 |
Pension Benefit Plan [Member] | Large Cap Value [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Available-for-sale securities | 309 | 249 |
Pension Benefit Plan [Member] | Balanced Fund [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Available-for-sale securities | 898 | 745 |
Pension Benefit Plan [Member] | Balanced Fund [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Available-for-sale securities | $ 898 | $ 745 |

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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562 + Details
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- Definition Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. + References No definition available. + Details
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