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Financial instruments and risk management - Summary of credit ratings (Parenthetical) (Detail) - Capital and liquidity risk management [member] |  | 12 Months Ended
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Feb. 12, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Long term rating [member] |  |  |
Disclosure of external credit grades [Line Items] |  |  |
Credit ratings |  | A-/A3 | A-/Baa1
Long term rating [member] | Change in credit rating [member] |  |  |
Disclosure of external credit grades [Line Items] |  |  |
Credit ratings | A |  |
Outlook [member] |  |  |
Disclosure of external credit grades [Line Items] |  |  |
Credit ratings |  | (a)Positive/Stable | Stable/ Stable
Outlook [member] | Change in credit rating [member] |  |  |
Disclosure of external credit grades [Line Items] |  |  |
Credit ratings | stable |  |

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- Definition The description of the information used to assess the credit quality of financial assets with credit risk that are neither past due nor impaired that the entity does not separately disclose. [Refer: Credit risk [member]] + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Note Expiry date 2018-01-01 -Name IFRS -Number 7 -IssueDate 2016-01-13 -Paragraph IG23 -Subparagraph d -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2016-03-30&anchor=para_IG23_d&doctype=Implementation%20Guidance -URIDate 2016-03-31 + Details
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- Definition Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table. + References No definition available. + Details
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- Details
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- Details
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- Details
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- Details
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