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Retirement Plans - Schedule of Amounts Recognized in Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | --- | ---
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |
Net (gain) loss | $ (243) | $ (366) | $ (461)
Transition obligation | 424 | 0 | 0
Total | 181 | $ (366) | $ (461)
Postretirement Benefit Costs [Member] |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |
Net (gain) loss | (27) |  |
Transition obligation | 76 |  |
Total | $ 49 |  |

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