Stock-based compensation (Employee Share Investment Plan) (Details 10) shares in Millions, CAD in Millions	12 Months Ended
Dec. 31, 2016 CAD participants shares	Dec. 31, 2015 CAD participants shares	Dec. 31, 2014 CAD participants shares
Share Based Compensation Arrangement By Employee Share Investment Plan Disclosures [Line Items]
Expense for Company contribution during period under employee share investment plan	CAD 76	CAD 61	CAD 161
Employee Share Investment Plan [Member]
Share Based Compensation Arrangement By Employee Share Investment Plan Disclosures [Line Items]
Maximum percentage of gross salary employee can contribute toward purchase of Company common stock (in hundredths)	10.00%
Maximum percentage of additional contribution made by the Company of amount invested by employee (in hundredths)	35.00%
Maximum percentage of gross salary employer can contribute toward purchase of Company common stock (in hundredths)	6.00%
Number of employees participating in plan and holding shares at balance sheet date \| participants	19,108	19,728	18,488
Share purchases by Share Trusts (in shares) \| shares	1.9	2.0	2.1
Expense for Company contribution during period under employee share investment plan	CAD 37	CAD 38	CAD 34

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The number of employees participating in the plan and holding shares at the balance sheet date.

+ References

No definition available.

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The Company will invest, on the employee's behalf, this percentage of the amount invested by the employees, up to 6 % of the employee's gross salary.

+ References

No definition available.

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The highest percentage of annual salary that the employer will contribute toward purchase with respect to the plan.

+ References

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- Definition

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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The highest percentage of annual salary that an employee is permitted to utilize with respect to the plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

The number of shares purchased on the open market during the period for issuance to employees under the plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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