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Components of Retirement Plan Costs (Detail) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2018 | Mar. 31, 2017
Pension Plans |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Service Cost | $ 848 | $ 824
Interest Cost | 1,469 | 1,514
Expected Return on Plan Assets | (1,946) | (1,819)
Prior Service Cost Amortization | 81 | 66
Actuarial Loss Amortization | 1,447 | 1,176
Sub-total | 1,899 | 1,761
Amounts Capitalized and Deferred | (720) | (662)
Net Periodic Benefit Cost Recognized | 1,179 | 1,099
Other Postretirement Benefit Plans, Defined Benefit |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Service Cost | 733 | 744
Interest Cost | 851 | 978
Expected Return on Plan Assets | (409) | (337)
Prior Service Cost Amortization | 327 | 350
Actuarial Loss Amortization | 346 | 524
Sub-total | 1,848 | 2,259
Amounts Capitalized and Deferred | (742) | (1,037)
Net Periodic Benefit Cost Recognized | 1,106 | 1,222
Supplemental Employee Retirement Plans, Defined Benefit |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Service Cost | 122 | 115
Interest Cost | 101 | 98
Prior Service Cost Amortization | 47 | 47
Actuarial Loss Amortization | 122 | 74
Sub-total | 392 | 334
Amounts Capitalized and Deferred | (113) | (99)
Net Periodic Benefit Cost Recognized | $ 279 | $ 235

X
- 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 net periodic benefit cost (credit) 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) -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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- Definition Amounts of pension and other post retirement benefit costs capitalized and deferred. + References No definition available. + Details
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- Definition The gross amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain or loss, prior service cost or credit, transition asset or obligation, and gain or loss due to settlements or curtailments). + References No definition available. + Details
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