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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, 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 Pension and Other Post Retirement Benefits Plans, Net Prior Service Cost Credit [Roll Forward] + References No definition available. + Details
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- Definition Pension and Other Postretirement Benefit Plans, Accumulated Net Gains Losses [Roll Forward] + References No definition available. + Details
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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://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=121641772&loc=SL7669686-108580 + Details
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- Definition 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). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=121641772&loc=SL7669686-108580 + Details
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- Definition 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). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=121641772&loc=SL7669646-108580 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=121641772&loc=d3e637-108580 + Details
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- Definition Amount, before tax, of reclassification adjustment from accumulated other comprehensive (income) loss for prior service cost (credit) of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 17A -URI http://asc.fasb.org/extlink&oid=121641772&loc=SL34724391-108580 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=121641772&loc=d3e689-108580 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=121641772&loc=d3e637-108580 + Details
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