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Employee benefits - Accumulated other comprehensive losses (Detail) - USD ($) $ in Thousands | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | --- | ---
Pension Plans |  |  |
Amounts recognized in accumulated other comprehensive losss: |  |  |
Net loss | $ 288,882 | $ 304,330 |
Accumulated other comprehensive loss | 288,882 | 304,330 | $ 290,327
OPEB Plan |  |  |
Amounts recognized in accumulated other comprehensive losss: |  |  |
Net loss | 21,472 | 5,720 |
Accumulated other comprehensive loss | $ 21,472 | $ 5,720 | $ 24,079

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