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Shares - Share purchase program (Details) - CAD CAD in Thousands					1 Months Ended	12 Months Ended
	May 27, 2016	Dec. 22, 2015	Aug. 14, 2015	Dec. 18, 2014	Jun. 30, 2015	Dec. 31, 2016	Dec. 31, 2015
Equity, Class of Treasury Stock [Line Items]
Shares retired during period (shares)						2,733,414	1,803,715
Common stock | Stock Purchase Program
Equity, Class of Treasury Stock [Line Items]
Shares retired during period (shares)	1,657,514	532,520		1,771,195	1,271,195	1,075,900	1,771,195
Increase (decrease) as a result of the retirement of shares	CAD (14,121)	CAD (4,500)			CAD (10,917)	CAD (9,237)
Percentage of issued and outstanding common shares purchased under Purchase Program	5.00%			5.00%
Retired through share purchase programs (in shares)			532,520
Additional paid in capital | Stock Purchase Program
Equity, Class of Treasury Stock [Line Items]
Increase (decrease) as a result of the retirement of shares	CAD 9,021	CAD 2,948			CAD 6,284	CAD 5,133

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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))
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Equity impact of the value of stock that has been repurchased and retired during the period. The excess of the purchase price over par value can be charged against retained earnings (once the excess is fully allocated to additional paid in capital).

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
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-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644

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