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Stock-based compensation - Share options plan (Details) - CAD CAD / shares in Units, CAD in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Proceeds from options exercised |  | CAD 575 | CAD 284
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] |  |  |
Exercised (in shares)) |  |  | (102,040)
Share option plan |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Number shares acquired per each option exercised |  | 1 |
Stock options annual vesting percentage |  | 20.00% |
Number of shares authorized for issuance (in shares) |  | 3,399,399 |
Proceeds from options exercised |  | CAD 575 | CAD 284
Total intrinsic value of options exercised |  | CAD 640 | CAD 187
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] |  |  |
Beginning balance (in shares) |  | 1,176,080 | 1,448,000
Exercised (in shares)) | [1] | (176,800) | (102,040)
Forfeited (in shares) |  | (85,740) | (169,880)
Ending balance (in shares) |  | 913,540 | 1,176,080
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward] |  |  |
Beginning balance (CAD per share) |  | CAD 5.56 | CAD 5.33
Exercised (CAD per share) | [1] | 3.26 | 2.77
Forfeited (CAD per share) |  | 12.36 | 5.31
Ending balance (CAD per share) |  | CAD 5.36 | CAD 5.56
Share option plan | Stock options |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Years from grant date stock options expire |  | 10 years |
[1] | All stock options exercised resulted in new common shares being issued (note 17(a))

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- Definition Amount of cash inflow from exercise of stock options granted under share-based compensation arrangement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3255-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=96867065&loc=SL79508275-113901 + Details
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- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Number of options outstanding, including both vested and non-vested options. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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