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Components of Retirement Plan Costs (Detail) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Pension Plans |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | $ 848 | $ 824 | $ 2,544 | $ 2,471
Interest Cost | 1,469 | 1,514 | 4,407 | 4,543
Expected Return on Plan Assets | (1,946) | (1,826) | (5,838) | (5,479)
Prior Service Cost Amortization | 81 | 66 | 243 | 197
Actuarial Loss Amortization | 1,447 | 1,165 | 4,341 | 3,496
Sub-total | 1,899 | 1,743 | 5,697 | 5,228
Amounts Capitalized and Deferred | (962) | (932) | (2,590) | (2,402)
Net Periodic Benefit Cost Recognized | 937 | 811 | 3,107 | 2,826
Other Postretirement Benefit Plans, Defined Benefit |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | 733 | 744 | 2,199 | 2,231
Interest Cost | 852 | 978 | 2,554 | 2,935
Expected Return on Plan Assets | (409) | (337) | (1,227) | (1,010)
Prior Service Cost Amortization | 327 | 350 | 981 | 1,049
Actuarial Loss Amortization | 346 | 524 | 1,038 | 1,573
Sub-total | 1,849 | 2,259 | 5,545 | 6,778
Amounts Capitalized and Deferred | (930) | (1,226) | (2,557) | (3,418)
Net Periodic Benefit Cost Recognized | 919 | 1,033 | 2,988 | 3,360
Supplemental Employee Retirement Plans, Defined Benefit |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | 122 | 115 | 366 | 345
Interest Cost | 101 | 98 | 303 | 294
Prior Service Cost Amortization | 47 | 47 | 141 | 141
Actuarial Loss Amortization | 121 | 74 | 365 | 222
Sub-total | 391 | 334 | 1,175 | 1,002
Amounts Capitalized and Deferred | (113) | (99) | (339) | (297)
Net Periodic Benefit Cost Recognized | $ 278 | $ 235 | $ 836 | $ 705

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