Major Commitments and Contingencies (Contingencies) (Details) - Personal Injury and Other Claims Provision [Member] - CAD CAD in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Canada [Member]
Contingencies
Increase (decrease) to personal injury and other claims provision persuant to the results of external actuarial studies	CAD 2	CAD (11)	CAD (12)
Loss Contingency Accrual [Roll Forward]
Beginning of year	183	191	203
Accruals and other	38	24	17
Payments	(38)	(32)	(29)
End of year	183	183	191
Current portion - End of year	40	39	27
U.S. [Member]
Contingencies
Increase (decrease) to personal injury and other claims provision persuant to the results of external actuarial studies	15	21	(5)
Loss Contingency Accrual [Roll Forward]
Beginning of year	118	105	95
Accruals and other	46	51	22
Payments	(41)	(34)	(30)
Foreign exchange	(7)	(4)	18
End of year	116	118	105
Current portion - End of year	CAD 25	CAD 37	CAD 24

The Increase or (Decrease) in the loss contingencies during the reporting period related to currency translations.

No definition available.

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- Definition

The increase (decrease) in the loss contingency accrual in the period from revisions in estimates to existing obligations.

+ References

No definition available.

+ Details

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- References

No definition available.

+ Details

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- Definition

Amount of loss contingency liability.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349

+ Details

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- Definition

Amount of loss contingency liability expected to be resolved within one year or the normal operating cycle, if longer.

+ References

+ Details

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- Definition

Amount of cash outflow reducing loss contingency liability.

+ References

+ Details

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- Definition

Amount charged against operating income increasing loss contingency liability, after adjustments to reduce previously estimated charges.

+ References

+ Details

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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