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Regulatory matters - Regulatory Assets and Liabilities (Detail) - USD ($) $ in Thousands | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2018 | Dec. 31, 2018 | Dec. 31, 2017
Regulatory Liabilities [Line Items] |  |  |
Total regulatory assets |  | $ 450,474 | $ 441,526
Less: current regulatory assets |  | (59,037) | (66,567)
Non-current regulatory assets |  | 391,437 | 374,959
Total regulatory liabilities |  | 578,592 | 576,124
Less: current regulatory liabilities |  | (39,005) | (37,687)
Non-current regulatory liabilities |  | 539,587 | 538,437
Reduction of regulatory asset | $ 15,586 |  | 327,947
Increase of regulatory liability |  |  | 327,947
Taxes |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory liabilities |  | 323,384 | 321,138
Cost of removal |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory liabilities |  | 193,564 | 184,188
Rate-base offset |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory liabilities |  | 10,900 | 13,214
Fuel and commodity costs adjustment |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory liabilities |  | 23,517 | 23,543
Deferred compensation received in relation to lost production |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory liabilities |  | 6,897 | 9,398
Deferred construction costs - fuel related |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory liabilities |  | 7,258 | 7,418
Pension and post-employment benefits |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory liabilities |  | 877 | 10,082
Other |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory liabilities |  | $ 12,195 | 7,143
Rate adjustment |  |  |
Regulatory Liabilities [Line Items] |  |  |
Collection period for services rendered |  | 24 months |
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 |  | $ 82,295 | 82,711
Regulatory assets recovery period |  | 7 years |
Pension and post-employment benefits |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory assets |  | $ 125,959 | 105,712
Regulatory asset approved not yet being recovered, average recovery term |  | 10 years |
Debt premium |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory assets |  | $ 48,847 | 57,406
Fuel and commodity costs adjustment |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory assets |  | 26,310 | 34,525
Rate adjustment |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory assets |  | 36,484 | 35,813
Clean Energy and other customer programs |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory assets |  | 22,269 | 20,582
Deferred construction costs - fuel related |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory assets |  | 13,986 | 14,344
Asset retirement |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory assets |  | 21,048 | 16,080
Taxes |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory assets |  | 34,822 | 36,546
Rate case costs |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory assets |  | 7,990 | 9,295
Other |  |  |
Regulatory Liabilities [Line Items] |  |  |
Total regulatory assets |  | $ 30,464 | $ 28,512

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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://www.xbrl.org/2003/role/presentationRef -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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