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Shares - Share purchase program (Details) - CAD ($) $ in Thousands | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Aug. 14, 2017 | Aug. 09, 2016
Equity, Class of Treasury Stock [Line Items] |  |  |  |
Shares purchased and subsequently cancelled during period (in shares) | (1,281,485) | (2,625,557) |  |
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) | (1,281,485.000) | (1,142,762) |  |
2016 Share Repurchase Program |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |
Maximum number of shares to be purchased (in shares) |  |  |  | 2,733,482
Shares purchased and subsequently cancelled during period (in shares) |  | (1,657,514) |  |
Common shares |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |
Shares purchased and subsequently cancelled during period (in shares) | (1,281,485) | (2,625,557) |  |
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) | $ (9,810) |  |
Common shares | 2016 Share Repurchase Program |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |
Shares purchased and subsequently cancelled during period (in shares) |  | (1,482,795) |  |
Increase (decrease) as a result of the retirement of shares |  | $ (12,763) |  |
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 | 4,119 |  |
Additional paid-in capital | 2016 Share Repurchase Program |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |
Increase (decrease) as a result of the retirement of shares |  | $ 3,484 |  |

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