Income Taxes - Narrative (Details) - CAD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Income Tax Disclosure [Abstract]
Effective tax rate	25.92%	25.60%
Foreign exchange loss (gain) on long-term debt	$ 49	$ (28)
Estimated annual effective tax rate	24.75%	26.50%
Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation and benefits		$ (51)

Effective Income Tax Rate Reconciliation, Estimated Rate, Excluding Discrete Items, Percent

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- Definition

The aggregate foreign currency transaction gain or loss on long-term debt (both realized and unrealized) included in determining net income for the reporting period net of hedging activity.

+ References

No definition available.

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- Definition

Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32698-109319

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(h)(2))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32687-109319

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Amount of cost (reversal of cost) for employee benefits and share-based payment arrangement. Includes, but is not limited to, pension, other postretirement, postemployment and termination benefits.

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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.

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