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Components of Retirement Plan Costs (Detail) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017
Pension Plans |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | $ 848 | $ 823 | $ 1,696 | $ 1,647
Interest Cost | 1,469 | 1,515 | 2,938 | 3,029
Expected Return on Plan Assets | (1,946) | (1,834) | (3,892) | (3,653)
Prior Service Cost Amortization | 81 | 65 | 162 | 131
Actuarial Loss Amortization | 1,447 | 1,155 | 2,894 | 2,331
Sub-total | 1,899 | 1,724 | 3,798 | 3,485
Amounts Capitalized and Deferred | (908) | (808) | (1,628) | (1,470)
Net Periodic Benefit Cost Recognized | 991 | 916 | 2,170 | 2,015
Other Postretirement Benefit Plans, Defined Benefit |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | 733 | 743 | 1,466 | 1,487
Interest Cost | 851 | 979 | 1,702 | 1,957
Expected Return on Plan Assets | (409) | (336) | (818) | (673)
Prior Service Cost Amortization | 327 | 349 | 654 | 699
Actuarial Loss Amortization | 346 | 525 | 692 | 1,049
Sub-total | 1,848 | 2,260 | 3,696 | 4,519
Amounts Capitalized and Deferred | (885) | (1,154) | (1,627) | (2,191)
Net Periodic Benefit Cost Recognized | 963 | 1,106 | 2,069 | 2,328
Supplemental Employee Retirement Plans, Defined Benefit |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | 122 | 115 | 244 | 230
Interest Cost | 101 | 98 | 202 | 196
Prior Service Cost Amortization | 47 | 47 | 94 | 94
Actuarial Loss Amortization | 122 | 74 | 244 | 148
Sub-total | 392 | 334 | 784 | 668
Amounts Capitalized and Deferred | (113) | (99) | (226) | (198)
Net Periodic Benefit Cost Recognized | $ 279 | $ 235 | $ 558 | $ 470

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