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Employee benefits - Accumulated other comprehensive losses (Detail) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
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Pension Plans |  |  |
Amounts recognized in accumulated other comprehensive losss: |  |  |
Net loss | $ 265,899 | $ 288,882 |
Accumulated other comprehensive loss | 265,899 | 288,882 | $ 304,330
OPEB Plan |  |  |
Amounts recognized in accumulated other comprehensive losss: |  |  |
Net loss | 32,152 | 21,472 |
Accumulated other comprehensive loss | $ 32,152 | $ 21,472 | $ 5,720

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