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Stock-based compensation - Options by exercise price range (Details) - Share option plan - CAD CAD / shares in Units, CAD in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Options outstanding, Number (in shares) | 913,540 |
Options outstanding, Weighted average remaining life | 4 years | 4 years 9 months
Options outstanding, Weighted average exercise price (in CAD per share) | CAD 5.36 | CAD 5.56
Total options exercisable (in shares) | 887,140 | 898,500
Options exercisable, Weighted average remaining life | 3 years 10 months 24 days |
Options exercisable, Weighted average exercise price (in CAD per share) | CAD 5.35 | CAD 6.21
Fair value of options vested | CAD 518 | CAD 594
Non-vested awards not yet recognized | CAD 46 | CAD 269
$2.75 |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Exercise price (CAD per share) | CAD 2.75 |
Options outstanding, Number (in shares) | 187,420 |
Options outstanding, Weighted average remaining life | 4 years 3 months 18 days |
Options outstanding, Weighted average exercise price (in CAD per share) | CAD 2.75 |
Options exercisable, Number (in shares) | 187,420 |
Options exercisable, Weighted average remaining life | 4 years 3 months 18 days |
Options exercisable, Weighted average exercise price (in CAD per share) | CAD 2.75 |
$2.79 |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Exercise price (CAD per share) | CAD 2.79 |
Options outstanding, Number (in shares) | 300,000 |
Options outstanding, Weighted average remaining life | 4 years 6 months |
Options outstanding, Weighted average exercise price (in CAD per share) | CAD 2.79 |
Options exercisable, Number (in shares) | 300,000 |
Options exercisable, Weighted average remaining life | 4 years 6 months |
Options exercisable, Weighted average exercise price (in CAD per share) | CAD 2.79 |
$3.69 |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Exercise price (CAD per share) | CAD 3.69 |
Options outstanding, Number (in shares) | 25,800 |
Options outstanding, Weighted average remaining life | 10 months 24 days |
Options outstanding, Weighted average exercise price (in CAD per share) | CAD 3.69 |
Options exercisable, Number (in shares) | 25,800 |
Options exercisable, Weighted average remaining life | 10 months 24 days |
Options exercisable, Weighted average exercise price (in CAD per share) | CAD 3.69 |
$5.91 |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Exercise price (CAD per share) | CAD 5.91 |
Options outstanding, Number (in shares) | 132,000 |
Options outstanding, Weighted average remaining life | 6 years |
Options outstanding, Weighted average exercise price (in CAD per share) | CAD 5.91 |
Options exercisable, Number (in shares) | 105,600 |
Options exercisable, Weighted average remaining life | 6 years |
Options exercisable, Weighted average exercise price (in CAD per share) | CAD 5.91 |
$6.56 |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Exercise price (CAD per share) | CAD 6.56 |
Options outstanding, Number (in shares) | 70,780 |
Options outstanding, Weighted average remaining life | 3 years 10 months 24 days |
Options outstanding, Weighted average exercise price (in CAD per share) | CAD 6.56 |
Options exercisable, Number (in shares) | 70,780 |
Options exercisable, Weighted average remaining life | 3 years 10 months 24 days |
Options exercisable, Weighted average exercise price (in CAD per share) | CAD 6.56 |
$8.28 |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Exercise price (CAD per share) | CAD 8.28 |
Options outstanding, Number (in shares) | 10,000 |
Options outstanding, Weighted average remaining life | 1 year 6 months |
Options outstanding, Weighted average exercise price (in CAD per share) | CAD 8.28 |
Options exercisable, Number (in shares) | 10,000 |
Options exercisable, Weighted average remaining life | 1 year 6 months |
Options exercisable, Weighted average exercise price (in CAD per share) | CAD 8.28 |
$8.58 |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Exercise price (CAD per share) | CAD 8.58 |
Options outstanding, Number (in shares) | 30,000 |
Options outstanding, Weighted average remaining life | 2 years 8 months 12 days |
Options outstanding, Weighted average exercise price (in CAD per share) | CAD 8.58 |
Options exercisable, Number (in shares) | 30,000 |
Options exercisable, Weighted average remaining life | 2 years 8 months 12 days |
Options exercisable, Weighted average exercise price (in CAD per share) | CAD 8.58 |
$9.33 |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Exercise price (CAD per share) | CAD 9.33 |
Options outstanding, Number (in shares) | 55,780 |
Options outstanding, Weighted average remaining life | 2 years 1 month 6 days |
Options outstanding, Weighted average exercise price (in CAD per share) | CAD 9.33 |
Options exercisable, Number (in shares) | 55,780 |
Options exercisable, Weighted average remaining life | 2 years 1 month 6 days |
Options exercisable, Weighted average exercise price (in CAD per share) | CAD 9.33 |
$10.13 |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Exercise price (CAD per share) | CAD 10.13 |
Options outstanding, Number (in shares) | 51,760 |
Options outstanding, Weighted average remaining life | 3 years |
Options outstanding, Weighted average exercise price (in CAD per share) | CAD 10.13 |
Options exercisable, Number (in shares) | 51,760 |
Options exercisable, Weighted average remaining life | 3 years |
Options exercisable, Weighted average exercise price (in CAD per share) | CAD 10.13 |
$16.46 |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Exercise price (CAD per share) | CAD 16.46 |
Options outstanding, Number (in shares) | 50,000 |
Options outstanding, Weighted average remaining life | 3 months 18 days |
Options outstanding, Weighted average exercise price (in CAD per share) | CAD 16.46 |
Options exercisable, Number (in shares) | 50,000 |
Options exercisable, Weighted average remaining life | 3 months 18 days |
Options exercisable, Weighted average exercise price (in CAD per share) | CAD 16.46 |
Stock options |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Period for award recognition | 1 year | 11 months

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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + 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 (i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Unrecognized cost of unvested options awarded to employees as compensation. + 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 (i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Number of fully vested and expected to vest exercisable options that may be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur. + 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 (e)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The number of shares reserved for issuance pertaining to the outstanding exercisable stock options as of the balance sheet date in the customized range of exercise prices for which the market and performance vesting condition has been satisfied. + 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)(iii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Weighted average exercise price as of the balance sheet date for those equity-based payment arrangements exercisable and outstanding. + 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)(iii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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