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Components of Retirement Plan Costs (Detail) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Pension Plans |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | $ 3,295 | $ 3,402 | $ 3,689
Interest Cost | 6,057 | 5,945 | 5,392
Expected Return on Plan Assets | (7,306) | (7,257) | (6,779)
Prior Service Cost Amortization | 263 | 263 | 265
Actuarial Loss Amortization | 4,662 | 4,398 | 4,714
Sub-total | 6,971 | 6,751 | 7,281
Amounts Capitalized or Deferred | (3,122) | (3,008) | (3,397)
NPBC Recognized | 3,849 | 3,743 | 3,884
Other Postretirement Benefit Plans, Defined Benefit |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | 2,974 | 2,610 | 2,622
Interest Cost | 3,913 | 3,232 | 2,918
Expected Return on Plan Assets | (1,347) | (1,205) | (1,093)
Prior Service Cost Amortization | 1,399 | 1,486 | 1,682
Actuarial Loss Amortization | 2,098 | 1,049 | 1,150
Sub-total | 9,037 | 7,172 | 7,279
Amounts Capitalized or Deferred | (4,515) | (3,351) | (3,423)
NPBC Recognized | 4,522 | 3,821 | 3,856
Supplemental Employee Retirement Plans, Defined Benefit |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | 460 | 162 | 120
Interest Cost | 392 | 386 | 330
Prior Service Cost Amortization | 189 | 189 | 85
Actuarial Loss Amortization | 295 | 375 | 327
Sub-total | 1,336 | 1,112 | 862
NPBC Recognized | $ 1,336 | $ 1,112 | $ 862

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- Definition Amount of gain (loss) recognized in net periodic benefit (cost) credit of defined benefit plan. + 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 (h)(4) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of prior service cost (credit) recognized in net periodic benefit cost (credit) of defined benefit plan. + 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 (h)(5) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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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 Amount of expected return (loss) recognized in net periodic benefit (cost) credit, calculated based on expected long-term rate of return and market-related value of plan assets of defined benefit plan. + 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 (h)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of cost recognized for passage of time related to defined benefit plan. + 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 (h)(2) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit plan. + 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 (h)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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