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Stock-based compensation - Performance and deferred stock unit plan (Details) | 12 Months Ended
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Dec. 31, 2020 CAD ($) $ / shares shares | Dec. 31, 2019 CAD ($) $ / shares shares | Dec. 31, 2019 CAD ($) $ / shares $ / shares
Performance restricted share units (PSUs) |  |  |
Number of units |  |  |
Beginning balance (in shares) | 481,907 | 741,117 |
Granted (in shares) | 211,754 | 122,274 |
Vested/redeemed (in shares) | (201,104) | (372,924) |
Forfeited (in shares) |  | (8,560) |
Ending balance (in shares) | 492,557 | 481,907 |
Weighted-average exercise price $ per share |  |  |
Outstanding, beginning of period (CAD per unit) | $ / shares | $ 8.85 | $ 5.92 |
Granted, Weighted average exercise price (CAD per unit) | $ / shares | 8.55 | 13.12 |
Vested/redeemed, Weighted average exercise price (CAD per unit) | $ / shares | 8.51 | 4.45 |
Forfeited, Weighted average exercise price (CAD per unit) | $ / shares |  |  | $ 8.09
Outstanding, end of period (CAD per unit) | $ / shares | $ 8.86 | $ 8.85 |
Vested (in shares) | 201,104 | 372,924 |
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract] |  |  |
Risk-free interest rate | 0.30% | 1.43% |
Expected volatility | 48.71% | 40.12% |
Performance restricted share unit plan | Performance restricted share units (PSUs) |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Performance restricted share unit plan granted to the services to be provided | 2 years |  |
Award vesting period | 3 years |  |
Equity classified restricted share unit plan | Performance restricted share units (PSUs) |  |  |
Number of units |  |  |
Vested/redeemed (in shares) | (201,104) | (372,924) |
Weighted-average exercise price $ per share |  |  |
Total unrecognized compensation costs related to non-vested non-option share-based payment arrangements | $ | $ 3,405,000 | $ 2,905,000 | $ 2,905,000
Period for award recognition | 1 year 7 months 6 days | 1 year 2 months 12 days |
Vested (in shares) | 201,104 | 372,924 |
Settled (in shares) |  | 334,244 |
Settlement ratio, per PSU (in shares) | 2.00 | 2.00 |
Deferred share unit plan | Deferred stock units (DSUs) |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract] |  |  |
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) | 901,045 | 1,126,239 |
Granted (in shares) | 114,020 | 82,191 |
Vested/redeemed (in shares) | (9,562) | (307,385) |
Ending balance (in shares) | 1,005,503 | 901,045 |
Weighted-average exercise price $ per share |  |  |
Total unrecognized compensation costs related to non-vested non-option share-based payment arrangements | $ | $ 0 |  |
Vested (in shares) | 9,562 | 307,385 |
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract] |  |  |
Fair market value (CAD per share) | $ / shares | $ 12.42 | $ 15.95 | $ 15.95
Award units settled during the period | $ | $ 103,000 | $ 5,084,000 |
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 | $ | 1,728,000 | 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 | $ | $ 10,761,000 | $ 14,375,000 | $ 14,375,000

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- Definition 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). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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