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Pension and other post-retirement benefits - Amounts Recognized in Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022
Pension benefits |  |
Actuarial losses (gains) |  |
Balance, January 1 | $ (671) | $ 15,807
Additions to AOCI | (12,600) | (47,473)
Amortization in current period | 617 | (3,429)
Amortization due to plan settlements |  | 15
Recognition of settlement gain | 235 |
Reclassification to regulatory accounts | 5,517 | 34,409
Ending balance, December 31 | (6,902) | (671)
Past service losses (gains) |  |
Beginning balance, January 1 | (3,363) | (4,195)
Additions to AOCI | 0 | 0
Amortization in current period | 1,491 | 1,584
Amortization due to plan settlements |  | 0
Reclassification to regulatory accounts | (755) | (752)
Recognition of settlement gain | 0 |
Ending balance, December 31 | (2,627) | (3,363)
OPEB |  |
Actuarial losses (gains) |  |
Balance, January 1 | (33,550) | (15,630)
Additions to AOCI | (23,797) | (41,527)
Amortization in current period | 2,554 | 56
Amortization due to plan settlements |  | 0
Recognition of settlement gain | 0 |
Reclassification to regulatory accounts | 19,518 | 23,551
Ending balance, December 31 | (35,275) | (33,550)
Past service losses (gains) |  |
Beginning balance, January 1 | (2,190) | 310
Additions to AOCI | 853 | (24)
Amortization in current period | 0 | (2,476)
Amortization due to plan settlements |  | 0
Reclassification to regulatory accounts | 0 | 0
Recognition of settlement gain | 0 |
Ending balance, December 31 | $ (1,337) | $ (2,190)

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- Definition Amount, before tax, of accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -Publisher FASB -URI https://asc.fasb.org//1943274/2147482790/220-10-45-14A Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (j) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Amount, before tax, of gain (loss) for (increase) decrease in value of benefit obligation for change in actuarial assumptions and increase (decrease) in value of plan assets from experience different from that assumed of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 220 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 11 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482790/220-10-45-11 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (i) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 45 -Paragraph 10A -Subparagraph (i) -SubTopic 10 -Topic 220 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482790/220-10-45-10A + Details
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