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Pension and other post-retirement benefits - Amounts Recognized in Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Pension Plans
Pension and Other Postretirement Benefit Plans, Accumulated Net Gains Losses [Roll Forward]
Beginning balance, January 1	$ 38,510	$ 34,257
Additions to AOCI	50,026	17,905
Amortization in current period	(5,430)	(3,530)
Reclassification to regulatory accounts	(25,875)	(10,122)
Ending balance, December 31	57,231	38,510
Pension and Other Post Retirement Benefits Plans, Net Prior Service Cost Credit [Roll Forward]
Beginning balance, January 1	(6,180)	(6,221)
Additions to AOCI	(191)	(7,705)
Amortization in current period	1,609	784
Reclassification to regulatory accounts	(544)	6,962
Ending balance, December 31	(5,306)	(6,180)
Other Postretirement Benefit Plans, Defined Benefit
Pension and Other Postretirement Benefit Plans, Accumulated Net Gains Losses [Roll Forward]
Beginning balance, January 1	(9,146)	(13,888)
Additions to AOCI	22,036	14,871
Amortization in current period	(509)	409
Reclassification to regulatory accounts	(16,680)	(10,538)
Ending balance, December 31	(4,299)	(9,146)
Pension and Other Post Retirement Benefits Plans, Net Prior Service Cost Credit [Roll Forward]
Beginning balance, January 1	0	(208)
Additions to AOCI	0	0
Amortization in current period	0	208
Reclassification to regulatory accounts	0	0
Ending balance, December 31	$ 0	$ 0

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- Definition

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

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

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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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-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
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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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