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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
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

Pension And Other Employee Benefit Plans [Line Items]

+ References

No definition available.

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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

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