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Stock-based compensation - Options by exercise price range (Details)	12 Months Ended
	Dec. 31, 2020 $ / shares	Dec. 31, 2020 CAD ($) shares	Dec. 31, 2019 CAD ($) shares	Dec. 31, 2020 CAD ($) $ / shares shares	Dec. 31, 2019 $ / shares	Dec. 31, 2019 CAD ($) $ / shares shares
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options outstanding, Number (in shares) | shares				125,000
Options outstanding, Weighted average remaining life		1 year 10 months 24 days
Options outstanding, Weighted average exercise price (in CAD per share)				$ 4.16
Compensation costs related to non-vested awards not yet recognized | $				$ 0		$ 0
Stock options granted | shares		0	0
$2.75
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Exercise price (CAD per share)	$ 2.75
Options outstanding, Number (in shares) | shares				73,400
Options outstanding, Weighted average remaining life		1 year 8 months 12 days
Options outstanding, Weighted average exercise price (in CAD per share)	2.75
$5.91
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Exercise price (CAD per share)	5.91
Options outstanding, Number (in shares) | shares				31,100
Options outstanding, Weighted average remaining life		3 years
Options outstanding, Weighted average exercise price (in CAD per share)	5.91
$6.56
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Exercise price (CAD per share)	6.56
Options outstanding, Number (in shares) | shares				20,500
Options outstanding, Weighted average remaining life		10 months 24 days
Options outstanding, Weighted average exercise price (in CAD per share)	$ 6.56
Share option plan
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options outstanding, Weighted average remaining life			2 years 4 months 24 days
Options outstanding, Weighted average exercise price (in CAD per share)						$ 4.61
Options exercisable, Weighted average exercise price (in CAD per share) | (per share)				$ 4.16	$ 4.61
Fair value of options vested | $		$ 0	$ 0
Total options exercisable (in shares) | shares				125,000		238,600

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

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Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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Gross number of share options (or share units) granted during the period.

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

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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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