Stock-based compensation - Performance and deferred stock unit plan (Details)		12 Months Ended
Jul. 01, 2014	Dec. 31, 2018 CAD ($) $ / shares shares	Dec. 31, 2017 CAD ($) $ / shares shares
Performance restricted share units (PSUs)
Number of units
Beginning balance (in shares)		896,633	739,300
Granted (in shares)		197,763	248,824
Vested/redeemed (in shares)		(353,279)	(69,949)
Forfeited (in shares)			(21,542)
Ending balance (in shares)		741,117	896,633
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Intrinsic Value, Amount Per Share [Abstract]
Outstanding, beginning of period (CAD per unit) \| $ / shares		$ 4.81	$ 4.84
Granted, Weighted average exercise price (CAD per unit) \| $ / shares		8.01	5.98
Vested, Weighted average exercise price (CAD per unit) \| $ / shares		4.27	8.57
Forfeited, Weighted average exercise price (CAD per unit) \| $ / shares			4.05
Outstanding, end of period (CAD per unit) \| $ / shares		$ 5.92	$ 4.81
Settlement ratio, per PSU (in shares)		2.00	1.34
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Risk-free interest rate		1.98%	1.17%
Expected volatility		45.04%	46.47%
Performance restricted share unit plan \| Performance restricted share units (PSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years
July 2014 grant \| Performance restricted share units (PSUs)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Intrinsic Value, Amount Per Share [Abstract]
Total unrecognized compensation costs related to non-vested non-option share-based payment arrangements \| $		$ 2,713,000	$ 2,250,000
Period for award recognition		1 year 3 months 15 days	1 year 4 months 10 days
Deferred share unit plan \| Deferred stock units (DSUs)
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Annual fixed remuneration		50.00%
Percentage of annual bonus eligible for deferred stock units		50.00%
Liability classified deferred stock unit plan \| Deferred stock units (DSUs)
Number of units
Beginning balance (in shares)		991,583	941,937
Granted (in shares)		134,656	114,895
Vested/redeemed (in shares)		0	(65,249)
Ending balance (in shares)		1,126,239	991,583
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Intrinsic Value, Amount Per Share [Abstract]
Total unrecognized compensation costs related to non-vested non-option share-based payment arrangements \| $		$ 0
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Fair market value (CAD per share) \| $ / shares		$ 11.91	$ 5.72
Award units settled during the period \| $		$ 0	$ 343,000
Liability classified deferred stock unit plan \| Deferred stock units (DSUs) \| Accrued liabilities
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Current portion of award obligation \| $		0	0
Liability classified deferred stock unit plan \| Deferred stock units (DSUs) \| Other liabilities
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Non-current portion of award obligation \| $		$ 13,413,000	$ 5,672,000

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Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Percentage of Non-Officer Director Compensation Paid in the Form of Deferred Stock Units

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Share-based Compensation. Settlement Ratio, Common Shares Per PSU

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Aggregate carrying value as of the balance sheet date of the liabilities for stock option plans and other equity-based compensation arrangements payable within one year (or the operating cycle, if longer).

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Aggregate carrying value as of the balance sheet date of the liabilities for stock option plans and other equity-based compensation arrangements payable after one year (or the operating cycle, if longer).

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Aggregate cash paid by the entity during the period to settle equity instruments granted under equity-based payment arrangements.

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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.

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Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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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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