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Stock-based compensation - Textuals (Details) (CAD) | 12 Months Ended | 0 Months Ended | 12 Months Ended |  |  |  |  | 12 Months Ended |  |
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Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2013 Share option plan | Dec. 31, 2012 Share option plan | Dec. 31, 2013 Share option plan Stock options | Dec. 31, 2012 Share option plan Stock options | Sep. 22, 2010 Senior executive stock option plan | Dec. 31, 2013 Restricted share unit plan Restricted share units (RSUs) | Dec. 31, 2012 Restricted share unit plan Restricted share units (RSUs) | Dec. 31, 2013 Restricted share unit plan Restricted share units (RSUs) Accrued liabilities | Dec. 31, 2012 Restricted share unit plan Restricted share units (RSUs) Accrued liabilities | Dec. 31, 2013 Restricted share unit plan Restricted share units (RSUs) Other long-term liabilities | Dec. 31, 2012 Restricted share unit plan Restricted share units (RSUs) Other long-term liabilities | Dec. 31, 2013 Directors' deferred share unit plan Deferred stock units (DSUs) | Dec. 31, 2012 Directors' deferred share unit plan Deferred stock units (DSUs) | Dec. 31, 2013 Directors' deferred share unit plan Deferred stock units (DSUs) Accrued liabilities | Dec. 31, 2013 Directors' deferred share unit plan Deferred stock units (DSUs) Other long-term liabilities | Dec. 31, 2012 Directors' deferred share unit plan Deferred stock units (DSUs) Other long-term liabilities
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number shares acquired per each option exercised |  |  | 1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Years from grant date stock options expire |  |  |  |  | 10 years |  |  |  |  |  |  |  |  |  |  |  |  |
Stock options annual vesting percentage |  |  | 20.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from stock options exercised | 1,175,000 | 7,000 | 1,175,000 | 7,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total intrinsic value of options exercised |  |  | 524,000 | 5,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Options outstanding, weighted average remaining life |  |  | 6 years 11 months | 7 years 3 months |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value of options vested during the year |  |  | 1,278,000 | 1,436,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of options exercisable |  |  | 892,700 | 1,106,274 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Options exercisable, weighted average exercise price |  |  | 8.00 | 8.59 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total compensation costs related to non-vested awards not yet recognized |  |  | 1,762,000 | 2,983,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total compensation costs related to non-vested awards not yet recognized, weighted average recognition period |  |  |  |  | 3 years 2 months 12 days | 3 years 8 months 12 days |  | 1 year 3 months 4 days | 1 year 6 months 36 days |  |  |  |  |  |  |  |  |
Stock options reclassified from equity to a long-term liability |  |  |  |  |  |  | 550,000 |  |  |  |  |  |  |  |  |  |  |
Compensation cost related to the senior executive stock option plan transferred from additional paid-in capital to the senior executive stock option liability |  |  |  |  |  |  | 2,237,000 |  |  |  |  |  |  |  |  |  |  |
Service period of award subsequent to year of grant |  |  |  |  |  |  |  | 2 years |  |  |  |  |  |  |  |  |  |
Award vesting period |  |  |  |  |  |  |  | 3 years |  |  |  |  |  |  |  |  |  |
Current portion of award obligation |  |  |  |  |  |  |  | 1,933,000 | 503,000 | 1,933,000 | 503,000 |  |  | 0 | 253,000 | 0 |  |
Non-current portion of award obligation |  |  |  |  |  |  |  |  |  |  |  | 1,734,000 | 1,239,000 |  |  |  | 3,662,000 | 1,817,000
Award units settled during the period (units) |  |  |  |  |  |  |  | 154,330 | 345,679 |  |  |  |  | 179,831 | 63,413 |  |  |
Award units settled during the period |  |  |  |  |  |  |  | 727,000 | 1,677,000 |  |  |  |  | 968,000 | 175,000 |  |  |
Weighted average remaining contractual life of award |  |  |  |  |  |  |  | 1 year 3 months 4 days | 1 year 6 months 36 days |  |  |  |  |  |  |  |  |
Fair market value (CAD per share) |  |  |  |  |  |  |  | 6.24 |  |  |  |  |  | 6.24 | 3.31 |  |  |
Total unrecognized compensation costs related to non-vested share-based payment arrangements |  |  |  |  |  |  |  | 2,958,000 | 1,980,000 |  |  |  |  | 0 |  |  |  |
Percentage of non-officer directors' fixed remuneration in the form of DDSUs (50% or more) |  |  |  |  |  |  |  |  |  |  |  |  |  | 50.00% |  |  |  |
Number of trading days used to determine weighted average trading price of common shares |  |  |  |  |  |  |  | 5 days |  |  |  |  |  | 5 days |  |  |  |

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- Definition Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares. + 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 (d)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Weighted average exercise price as of the balance sheet date for those equity-based payment arrangements exercisable and outstanding. + 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 (c)(1)(iii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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