Stock-based Compensation (Employee Share Investment Plan) (Details) shares in Millions, CAD in Millions	12 Months Ended
Dec. 31, 2017 CAD participants shares	Dec. 31, 2016 CAD participants shares	Dec. 31, 2015 CAD participants shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares purchased on behalf of employees \| shares	0.5	0.7	1.4
Expense for Company contribution \| CAD	CAD 75	CAD 76	CAD 61
Employee Share Investment Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum percentage of gross salary eligible employees can subscribe to purchase shares of Company's common stock on the open market	10.00%
Maximum percentage of additional contribution made by the Company of amount invested by employee	35.00%
Maximum percentage of gross salary employer can contribute toward purchase of Company's common stock	6.00%
Number of participants holding shares \| participants	19,642	19,108	19,728
Number of shares purchased on behalf of employees \| shares	1.7	1.9	2.0
Expense for Company contribution \| CAD	CAD 36	CAD 37	CAD 38

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

No definition available.

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- Definition

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.

+ Details

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- Definition

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

+ References

No definition available.

+ Details

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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

+ Details

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Definition

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

+ Details

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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

Reference 2: 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

+ Details

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- Details

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