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Shares - Voting common shares (Details) - CAD CAD in Thousands |  |  |  |  |  | 12 Months Ended
--- | --- | --- | --- | --- | --- | ---
Aug. 14, 2017 | May 25, 2017 | Mar. 28, 2017 | Aug. 09, 2016 | Mar. 21, 2016 | Dec. 31, 2017 | Dec. 31, 2016
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |  |  |  |
Beginning balance, issued and outstanding (in shares) |  |  |  |  |  |  | 30,518,907 | 33,150,281
Beginning balance, common share outstanding net of treasury shares (in shares) |  |  |  |  |  |  | 28,305,660 | 31,893,478
Issued upon exercise of stock options (in shares) |  |  |  |  |  |  |  | 102,040
Purchase of treasury shares for settlement of certain equity classified stock-based compensation (in shares) |  |  |  |  |  |  | 353,592 | 87,554
Retired through share purchase programs (in shares) |  | (1,142,762) | (663,400) | (819,395) | (1,075,900) | (1,657,514) | (2,625,557.000) | (2,733,414)
Ending balance, issued and outstanding (in shares) |  |  |  |  |  |  | 28,070,150 | 30,518,907
Ending balance, common share outstanding net of treasury shares (in shares) |  |  |  |  |  |  | 25,452,224 | 28,305,660
Equity classified stock-based compensation settled (in shares) |  |  |  |  |  |  | (161,285) | (35,313)
Equity classified stock-based compensation settled |  |  |  |  |  |  | CAD 987 | CAD 130
Share option plan |  |  |  |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |  |  |  |
Issued upon exercise of stock options (in shares) | [1] |  |  |  |  |  | 176,800 | 102,040
Treasury shares |  |  |  |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |  |  |  |
Beginning balance, issued and outstanding (in shares) |  |  |  |  |  |  | 2,213,247 | 1,256,803
Purchase of treasury shares for settlement of certain equity classified stock-based compensation (in shares) |  |  |  |  |  |  | (758,271) | (1,043,998)
Purchase of treasury shares for settlement of certain equity classified stock-based compensation (in shares) |  |  |  |  |  |  | 353,592 | 87,554
Ending balance, issued and outstanding (in shares) |  |  |  |  |  |  | 2,617,926 | 2,213,247
[1] | All stock options exercised resulted in new common shares being issued (note 17(a))

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- Definition Common Stock, Shares, Outstanding, Net of Treasury Shares + References No definition available. + Details
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- Definition Stock Issued During Period, Shares, Treasury Stock Reissued to Satisfy Shared-based Compensation Obligations + References No definition available. + Details
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- Definition The amount of adjustment to stockholders' equity associated with an employee's income tax withholding obligation as part of a net-share settlement of a share-based award. + References No definition available. + Details
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- Definition Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(29)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition For net-share settlement of share-based awards when the employer settles employees' income tax withholding obligations, this element represents the number of shares the employees use to repay the employer. + References No definition available. + Details
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- Definition Number of share options (or share units) exercised during the current period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(29)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21463-112644 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(28)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Number of shares that have been repurchased and retired during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(28)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21463-112644 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(29)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Number of shares that have been repurchased during the period and are being held in treasury. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(29)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21463-112644 + Details
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