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Share capital - Narrative (Details) $ in Thousands |  |  | 2 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Oct. 12, 2007 USD ($) | Jun. 29, 2007 shares | Mar. 12, 2019 USD ($) | Mar. 14, 2018 shares | Dec. 31, 2018 USD ($) shares year | Dec. 31, 2018 CAD ($) shares year | Dec. 31, 2017 USD ($) shares year | Dec. 31, 2017 CAD ($) shares year | Dec. 31, 2016 shares
Disclosure of classes of share capital [line items] |  |  |  |  |  |  |  |  |
Shares issued on exercise of stock options |  |  |  |  | $ 22,397 |  | $ 25,658 |  |
Number of options exercised (in shares) | shares |  |  |  |  | 12,347,000 | 12,347,000 | 13,708,000 | 13,708,000 |
Common shares issued for cash on exercise of stock options (Note 12) |  |  |  |  | $ 22,805 |  | $ 26,503 |  |
Number of options granted (in shares) | shares |  |  |  |  | 25,380,000 | 25,380,000 | 22,968,000 | 22,968,000 |
Risk free annual interest rate |  |  |  |  | 2.00% | 2.00% | 1.30% | 1.30% |
Dividend yield rate |  |  |  |  | 0.00% | 0.00% | 0.00% | 0.00% |
Weighted average share price for share options exercised during period at date of exercise (in Cdn.$per share) |  |  |  |  |  | $ 3.55 |  | $ 3.87 |
Incentive Plan |  |  |  |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |  |  |  |
Shares issued on exercise of stock options | $ 4,955 |  |  |  |  |  |  |  |
Number of options granted (in shares) | shares |  | 4,955,000 |  |  |  |  |  |  |
Stock Options |  |  |  |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |  |  |  |
Share-based payments expense |  |  |  |  | $ 18,000 |  | $ 14,000 |  |
Term of options granted (in years) |  |  |  |  | 5 years | 5 years | 5 years | 5 years |
Award vesting period (in years) |  |  |  |  | 3 years | 3 years | 3 years | 3 years |
Fair value of shares from share-based payment transactions |  |  |  |  | $ 26,000 |  | $ 27,000 |  |
Share-based payments expense, capitalized to mining interests |  |  |  |  | 3,000 |  | 3,000 |  |
Restricted Share Units |  |  |  |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |  |  |  |
Share-based payments expense |  |  |  |  | 2,000 |  | 4,000 |  |
Fair value of shares from share-based payment transactions |  |  |  |  | $ 4,000 |  | $ 5,000 |  |
Number of units granted (in shares) | shares |  |  |  |  | 1,455,000 | 1,455,000 | 1,793,000 | 1,793,000 |
Deferred Share Unit |  |  |  |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |  |  |  |
Share-based payments expense |  |  |  |  | $ 1,000 |  | $ 1,000 |  |
Fair value of shares from share-based payment transactions |  |  |  |  | $ 1,000 |  | $ 1,000 |  |
Measurement period |  |  |  |  | 5 days | 5 days |  |  |
Number of units granted (in shares) | shares |  |  |  |  | 291,000 | 291,000 | 225,000 | 225,000 |
Bottom of range |  |  |  |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |  |  |  |
Exercise price, share options granted (in Cdn.$per share) |  |  |  |  |  | $ 2.94 |  | $ 3.27 |
Expected volatility |  |  |  |  | 56.00% | 56.00% | 61.00% | 61.00% |
Top of range |  |  |  |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |  |  |  |
Exercise price, share options granted (in Cdn.$per share) |  |  |  |  |  | $ 3.44 |  | $ 4.05 |
Expected option life (in years) | year |  |  |  |  | 3 | 3 | 3 | 3 |
Expected volatility |  |  |  |  |  |  | 62.00% | 62.00% |
Vesting 1 year from grant | Restricted Share Units |  |  |  |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |  |  |  |
Award vesting percent |  |  |  |  | 33.33% | 33.33% |  |  |
Vesting 2 years from grant | Restricted Share Units |  |  |  |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |  |  |  |
Award vesting period (in years) |  |  |  |  | 2 years | 2 years |  |  |
Award vesting percent |  |  |  |  | 33.33% | 33.33% |  |  |
Vesting 3 years from grant | Restricted Share Units |  |  |  |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |  |  |  |
Award vesting period (in years) |  |  |  |  | 3 years | 3 years |  |  |
Award vesting percent |  |  |  |  | 33.33% | 33.33% |  |  |
Ordinary shares |  |  |  |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |  |  |  |
Number of shares outstanding (in shares) | shares |  |  |  |  | 994,621,917 |  |  |  |
Number of shares held in trust under the Company's Incentive Plan (in shares) | shares |  |  |  |  | 1,705,000 |  |  |  |
Preference shares |  |  |  |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |  |  |  |
Number of shares outstanding (in shares) | shares |  |  |  |  | 0 |  |  |  |
Share capital |  |  |  |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |  |  |  |
Number of shares outstanding (in shares) | shares |  |  |  |  | 994,622,000 |  | 980,933,000 |  | 964,892,000
Shares issued on exercise of stock options |  |  |  |  | $ 22,397 |  | $ 25,658 |  |
Number of options exercised (in shares) | shares |  |  |  |  | 12,173,000 | 12,173,000 | 13,343,000 | 13,343,000 |
Major ordinary share transactions |  |  |  |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |  |  |  |
Common shares issued for cash on exercise of stock options (Note 12) |  |  | $ 20,000 |  |  |  |  |  |
Major ordinary share transactions | Ordinary shares | Stock Options |  |  |  |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |  |  |  |
Number of options exercised (in shares) | shares |  |  |  | 10,000,000 |  |  |  |  |

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