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Regulatory matters - Regulatory Assets and Liabilities (Detail) - USD ($) $ in Thousands |  | 12 Months Ended
--- | --- | ---
Jun. 01, 2018 | Dec. 31, 2019 | Dec. 31, 2018
Regulatory Liabilities [Line Items] |  |  |
Total regulatory assets |  | $ 559,887 | $ 460,095
Less: current regulatory assets |  | (50,213) | (59,037)
Non-current regulatory assets |  | 509,674 | 401,058
Total regulatory liabilities |  | 598,062 | 588,213
Less: current regulatory liabilities |  | (41,683) | (39,005)
Non-current regulatory liabilities |  | 556,379 | 549,208
Reduction of regulatory asset | $ 15,586 |  |
Increase of regulatory liability |  |  | 15,586
Income Taxes |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory liabilities |  | 321,960 | 323,384
Cost of removal |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory liabilities |  | 196,423 | 193,564
Rate-base offset |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory liabilities |  | 8,719 | 10,900
Fuel and commodity costs adjustment |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory liabilities |  | 16,645 | 21,352
Rate adjustment mechanism |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory liabilities |  | $ 10,446 | 4,210
Collection period for services rendered |  | 24 months |
Deferred construction costs - fuel related |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory liabilities |  | $ 7,097 | 7,258
Pension and post-employment benefits |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory liabilities |  | 22,256 | 11,791
Other |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory liabilities |  | $ 14,516 | 15,754
Minimum | Rate-base offset |  |  |
Regulatory Liabilities [Line Items] |  |  |
Amortization period for regulatory liability |  | 10 years |
Maximum | Rate-base offset |  |  |
Regulatory Liabilities [Line Items] |  |  |
Amortization period for regulatory liability |  | 16 years |
Environmental remediation |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory assets |  | $ 82,300 | 82,295
Regulatory assets recovery period |  | 7 years |
Pension and post-employment benefits |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory assets |  | $ 143,292 | 135,580
Regulatory asset approved not yet being recovered, average recovery term |  | 10 years |
Income Taxes |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory assets |  | $ 71,506 | 34,822
Debt premium |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory assets |  | 42,150 | 48,847
Fuel and commodity costs adjustment |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory assets |  | 23,433 | 26,310
Rate adjustment mechanism |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory assets |  | 69,121 | 37,202
Clean Energy and other customer programs |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory assets |  | 26,369 | 24,095
Deferred construction costs - fuel related |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory assets |  | 38,833 | 13,986
Asset retirement obligation |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory assets |  | 23,841 | 21,048
Long-term maintenance contract |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory assets |  | 13,264 | 8,283
Rate case costs |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory assets |  | 6,695 | 6,164
Other |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory assets |  | $ 19,083 | $ 21,463
New Brunswick Gas | Rate adjustment mechanism |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory assets recovery period |  | 25 years |
New Brunswick Gas | Deferred Capitalized Costs |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory assets recovery period |  | 29 years |
Regulatory Asset Annual Recovery Rate, Percent |  | 2.43% |

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- Definition Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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