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Share capital (Summary of Changes in Company's Outstanding Stock Options) (Details) shares in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 CAD shares | Dec. 31, 2016 CAD shares
Number |  |
Balance, beginning of year | shares | 1,771 | 1,516
Granted | shares | 1,058 | 808
Exercised | shares | (261) | (93)
Expired | shares | (252) | (460)
Balance, end of year | shares | 2,316 | 1,771
Weighted average exercise price |  |
Balance, beginning of year | CAD | CAD 4.64 | CAD 9.5
Granted | CAD | 3.86 | 2.09
Exercised | CAD | 3.22 | 3.12
Expired | CAD | 17.31 | 16.5
Balance, end of year | CAD | CAD 3.06 | CAD 4.64

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