Stock-based compensation - Options by exercise price range (Details) - Share option plan - CAD CAD / shares in Units, CAD in Thousands	12 Months Ended
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

Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price

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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)
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Unrecognized cost of unvested options awarded to employees as compensation.

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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.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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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.

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The number of shares reserved for issuance pertaining to the outstanding stock options as of the balance sheet date for all option plans in the customized range of exercise prices.

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-Publisher FASB
-Name Accounting Standards Codification
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Fair value of options vested. Excludes equity instruments other than options, for example, but not limited to, share units, stock appreciation rights, restricted stock.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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- Definition

Weighted average exercise price as of the balance sheet date for those equity-based payment arrangements exercisable and outstanding.

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-Name Accounting Standards Codification
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Weighted average remaining contractual term of exercisable stock options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The weighted average price as of the balance sheet date at which grantees could acquire the underlying shares with respect to all outstanding stock options which are in the customized range of exercise prices.

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Weighted average remaining contractual term of outstanding stock options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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