Pension and other post-retirement benefits - Amounts Recognized in Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Thousands	12 Months Ended
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)

Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Loss On Plan Settlements, Before Tax

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Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment to Regulatory Accounts, before Tax

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Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Loss On Plan Settlements, Before Tax

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Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Recognition of Settlement Gain (Loss) On Plan Settlements, Before Tax

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Other Comprehensive Income (Loss), Defined Benefit Plan, Recognition of Settlement Gain (Loss), Before Tax

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- Definition

Pension and Other Post Retirement Benefits Plans, Net Prior Service Cost Credit [Roll Forward]

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Pension and Other Postretirement Benefit Plans, Accumulated Net Gains Losses [Roll Forward]

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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

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Amount, before tax, of accumulated other comprehensive (income) loss for cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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Amount, before tax, of cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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Amount, before tax, of reclassification adjustment from accumulated other comprehensive (income) loss for prior service cost (credit) of defined benefit plan.

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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.

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Amount, before tax, of reclassification adjustment from accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan.

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