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Components of Retirement Plan Costs (Detail) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016
Pension Plans |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | $ 824 | $ 851 | $ 2,471 | $ 2,553
Interest Cost | 1,514 | 1,486 | 4,543 | 4,458
Expected Return on Plan Assets | (1,826) | (1,814) | (5,479) | (5,442)
Prior Service Cost Amortization | 66 | 66 | 197 | 198
Actuarial Loss Amortization | 1,165 | 1,099 | 3,496 | 3,297
Sub-total | 1,743 | 1,688 | 5,228 | 5,064
Amounts Capitalized and Deferred | (932) | (839) | (2,402) | (2,246)
Net Periodic Benefit Cost Recognized | 811 | 849 | 2,826 | 2,818
Other Postretirement Benefit Plans, Defined Benefit |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | 744 | 652 | 2,231 | 1,956
Interest Cost | 978 | 808 | 2,935 | 2,424
Expected Return on Plan Assets | (337) | (301) | (1,010) | (903)
Prior Service Cost Amortization | 350 | 372 | 1,049 | 1,116
Actuarial Loss Amortization | 524 | 262 | 1,573 | 786
Sub-total | 2,259 | 1,793 | 6,778 | 5,379
Amounts Capitalized and Deferred | (1,226) | (904) | (3,418) | (2,508)
Net Periodic Benefit Cost Recognized | 1,033 | 889 | 3,360 | 2,871
Supplemental Employee Retirement Plans, Defined Benefit |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | 115 | 41 | 345 | 123
Interest Cost | 98 | 96 | 294 | 288
Prior Service Cost Amortization | 47 | 47 | 141 | 141
Actuarial Loss Amortization | 74 | 94 | 222 | 282
Sub-total | 334 | 278 | 1,002 | 834
Net Periodic Benefit Cost Recognized | $ 334 | $ 278 | $ 1,002 | $ 834

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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 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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