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Stock-based compensation - Restricted share unit plan (Details) - Restricted share units (RSUs) - CAD CAD / shares in Units, CAD in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Liability classified restricted share unit plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Period for recognition in years following grant	2 years
Award vesting period	3 years
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Beginning balance (in shares)	0	290,790
Vested (in shares)		(290,790)
Ending balance (in shares)		0
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Award units settled during the period		CAD 723
Equity classified restricted share unit plan
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Beginning balance (in shares)	1,123,975	774,156
Granted/issued (in shares)	355,292	501,523
Vested (in shares)	(259,860)	(87,580)
Forfeited (in shares)	(29,474)	(64,124)
Ending balance (in shares)	1,189,933	1,123,975
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Outstanding, beginning of period (CAD per unit)	CAD 4.20	CAD 4.45
Granted/Issued, Weighted average exercise price (CAD per unit)	6.02	3.78
Vested, Weighted average exercise price (CAD per unit)	6.24	3.51
Forfeited, Weighted average exercise price (CAD per unit)	4.00	3.50
Outstanding, end of period (CAD per unit)	CAD 4.22	CAD 4.20
Total unrecognized compensation costs related to non-vested non-option share-based payment arrangements	CAD 2,199	CAD 1,945
Period for award recognition	1 year 3 months 18 days	1 year 7 months 1 day

X

- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period Subsequent to Year of Grant

+ References

No definition available.

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

+ 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 (k)
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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.

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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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-Publisher FASB
-Name Accounting Standards Codification
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- Definition

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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- Definition

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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-Publisher FASB
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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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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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