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Regulatory matters - Regulatory Assets and Liabilities (Detail) - CAD CAD in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016
Regulatory Assets And Liabilities [Line Items] |  |
Total regulatory assets | CAD 551,134 | CAD 291,964
Less current regulatory assets | (83,508) | (48,440)
Non-current regulatory assets | 467,626 | 243,524
Total regulatory liabilities | 725,056 | 182,734
Less current regulatory liabilities | (47,278) | (47,769)
Non-current regulatory liabilities | 677,778 | 134,965
Reduction of regulatory asset | 411,409 |
Increase of regulatory liability | 411,409 |
Taxes |  |
Regulatory Assets And Liabilities [Line Items] |  |
Total regulatory liabilities | 402,868 | 1,501
Cost of removal |  |
Regulatory Assets And Liabilities [Line Items] |  |
Total regulatory liabilities | 231,064 | 110,330
Rate-base offset |  |
Regulatory Assets And Liabilities [Line Items] |  |
Total regulatory liabilities | 16,577 | 20,946
Fuel and commodity costs adjustment |  |
Regulatory Assets And Liabilities [Line Items] |  |
Total regulatory liabilities | 29,535 | 34,012
Deferred compensation received in relation to lost production |  |
Regulatory Assets And Liabilities [Line Items] |  |
Total regulatory liabilities | 11,789 | 0
Deferred construction costs - fuel related |  |
Regulatory Assets And Liabilities [Line Items] |  |
Total regulatory liabilities | 9,306 | 0
Pension and post-employment benefits |  |
Regulatory Assets And Liabilities [Line Items] |  |
Total regulatory liabilities | 12,648 | 5,481
Other |  |
Regulatory Assets And Liabilities [Line Items] |  |
Total regulatory liabilities | CAD 11,269 | 10,464
Rate adjustment |  |
Regulatory Assets And Liabilities [Line Items] |  |
Collection period for services rendered | 24 months |
Environmental remediation |  |
Regulatory Assets And Liabilities [Line Items] |  |
Total regulatory assets | CAD 103,761 | 104,160
Regulatory assets recovery period | 7 years |
Pension and post-employment benefits |  |
Regulatory Assets And Liabilities [Line Items] |  |
Total regulatory assets | CAD 132,615 | 75,527
Portion related to current acquisition | CAD 21,626 |
Regulatory asset approved not yet being recovered, average recovery term | 10 years |
Debt premium |  |
Regulatory Assets And Liabilities [Line Items] |  |
Total regulatory assets | CAD 72,016 | 25,173
Fuel and commodity costs adjustment |  |
Regulatory Assets And Liabilities [Line Items] |  |
Total regulatory assets | 43,311 | 6,990
Rate adjustment |  |
Regulatory Assets And Liabilities [Line Items] |  |
Total regulatory assets | 44,523 | 40,602
Clean Energy and other customer programs |  |
Regulatory Assets And Liabilities [Line Items] |  |
Total regulatory assets | 25,820 | 2,106
Deferred construction costs - fuel related |  |
Regulatory Assets And Liabilities [Line Items] |  |
Total regulatory assets | 17,994 | 0
Asset retirement |  |
Regulatory Assets And Liabilities [Line Items] |  |
Total regulatory assets | 20,172 | 2,113
Taxes |  |
Regulatory Assets And Liabilities [Line Items] |  |
Total regulatory assets | 45,847 | 10,182
Rate case costs |  |
Regulatory Assets And Liabilities [Line Items] |  |
Total regulatory assets | 11,660 | 8,572
Other |  |
Regulatory Assets And Liabilities [Line Items] |  |
Total regulatory assets | CAD 33,415 | CAD 16,539
Minimum | Rate-base offset |  |
Regulatory Assets And Liabilities [Line Items] |  |
Amortization period for regulatory liability | 10 years |
Maximum | Rate-base offset |  |
Regulatory Assets And Liabilities [Line Items] |  |
Amortization period for regulatory liability | 16 years |

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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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