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Shares - Share purchase program (Details) - CAD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 | Aug. 14, 2017
Equity, Class of Treasury Stock [Line Items] |  |
Shares purchased and subsequently cancelled during period (in shares) | 1,281,485 |
2017 Share Repurchase Program |  |
Equity, Class of Treasury Stock [Line Items] |  |
Maximum number of shares to be purchased (in shares) |  | 2,424,333
Shares purchased and subsequently cancelled during period (in shares) | 1,281,485 |
Common shares |  |
Equity, Class of Treasury Stock [Line Items] |  |
Shares purchased and subsequently cancelled during period (in shares) | 1,281,485 |
Common shares | 2017 Share Repurchase Program |  |
Equity, Class of Treasury Stock [Line Items] |  |
Increase (decrease) as a result of the retirement of shares | $ (10,975) |
Additional paid-in capital | 2017 Share Repurchase Program |  |
Equity, Class of Treasury Stock [Line Items] |  |
Increase (decrease) as a result of the retirement of shares | $ 1,435 |

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