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Pension and Other Postretirement Benefits - Fair Values of Pension Plan Assets by Asset Category (Detail) - USD ($) $ in Thousands | Dec. 31, 2016 | Dec. 31, 2015
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Pension And Other Employee Benefit Plans [Line Items] |  |
Fair values of pension plan assets by asset category | $ 182,370 | $ 204,372
Large-Cap Fund [Member] |  |
Pension And Other Employee Benefit Plans [Line Items] |  |
Fair values of pension plan assets by asset category | 65,495 | 77,618
Mid-Cap Fund [Member] |  |
Pension And Other Employee Benefit Plans [Line Items] |  |
Fair values of pension plan assets by asset category | 11,419 | 14,427
Small-Cap Fund [Member] |  |
Pension And Other Employee Benefit Plans [Line Items] |  |
Fair values of pension plan assets by asset category | 17,184 | 19,260
Government Bond Fund [Member] |  |
Pension And Other Employee Benefit Plans [Line Items] |  |
Fair values of pension plan assets by asset category | 26,151 | 26,827
Corporate Bond Fund [Member] |  |
Pension And Other Employee Benefit Plans [Line Items] |  |
Fair values of pension plan assets by asset category | 20,971 | 24,975
Fixed Income Fund [Member] |  |
Pension And Other Employee Benefit Plans [Line Items] |  |
Fair values of pension plan assets by asset category | 40,958 | 40,989
Cash & Equivalents [Member] |  |
Pension And Other Employee Benefit Plans [Line Items] |  |
Fair values of pension plan assets by asset category | 192 | 276
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 | 192 | 9,541
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 |  | 3,266
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 |  | 957
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 |  | 461
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 |  | 1,387
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 |  | 3,194
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 | 192 | 276
Significant Observable Inputs (Level 2) [Member] |  |
Pension And Other Employee Benefit Plans [Line Items] |  |
Fair values of pension plan assets by asset category | 40,958 | 40,989
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 | $ 40,958 | $ 40,989

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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 30 -Section 35 -Paragraph 50 -URI http://asc.fasb.org/extlink&oid=64937038&loc=d3e12355-114930 Reference 2: 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=65897934&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Plan Assets -URI http://asc.fasb.org/extlink&oid=6749337 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Plan Assets -URI http://asc.fasb.org/extlink&oid=6520858 + Details
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