Employee Future Benefits - Fair Value of Plan Assets (Details) - CAD CAD in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure of net defined benefit liability (asset) [line items]
Net asset, beginning of period	CAD (230)
Administration expenses	(2)	CAD (2)	CAD (2)
Net asset, ending of period	(247)	(230)
Plan assets
Disclosure of net defined benefit liability (asset) [line items]
Net asset, beginning of period	433	438
Interest on plan assets	15	16	16
Net return on plan assets	26	10
Contributions	12	18
Benefits paid	(55)	(46)
Effect of translation on US plans	(1)	(1)
Net asset, ending of period	428	433	438
Supplemental
Disclosure of net defined benefit liability (asset) [line items]
Net asset, beginning of period	(72)
Administration expenses	0	0	0
Net asset, ending of period	(75)	(72)
Supplemental \| Plan assets
Disclosure of net defined benefit liability (asset) [line items]
Net asset, beginning of period	10	9
Interest on plan assets	0	0	0
Net return on plan assets	0	0
Contributions	6	6
Benefits paid	(4)	(5)
Effect of translation on US plans	0	0
Net asset, ending of period	12	10	9
Registered
Disclosure of net defined benefit liability (asset) [line items]
Net asset, beginning of period	(131)
Administration expenses	(2)	(2)	(2)
Net asset, ending of period	(145)	(131)
Registered \| Plan assets
Disclosure of net defined benefit liability (asset) [line items]
Net asset, beginning of period	423	429
Interest on plan assets	15	16	16
Net return on plan assets	26	10
Contributions	6	11
Benefits paid	(51)	(40)
Effect of translation on US plans	(1)	(1)
Net asset, ending of period	416	423	429
Other
Disclosure of net defined benefit liability (asset) [line items]
Net asset, beginning of period	(27)
Administration expenses	0	0	0
Net asset, ending of period	(27)	(27)
Other \| Plan assets
Disclosure of net defined benefit liability (asset) [line items]
Net asset, beginning of period	0	0
Interest on plan assets	0	0	0
Net return on plan assets	0	0
Contributions	0	1
Benefits paid	0	(1)
Effect of translation on US plans	0	0
Net asset, ending of period	CAD 0	CAD 0	CAD 0

The decrease (increase) in the net defined benefit liability (asset) resulting from contributions to a defined benefit plan. [Refer: Net defined benefit liability (asset); Defined benefit plans [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2017-01-01
-Paragraph 141
-Subparagraph f
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_f&doctype=Standard
-URIDate 2017-03-09

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- Definition

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

+ References

No definition available.

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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from changes in foreign exchange rates. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2017-01-01
-Paragraph 141
-Subparagraph e
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_e&doctype=Standard
-URIDate 2017-03-09

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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from the passage of time. [Refer: Interest expense; Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2017-01-01
-Paragraph 141
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_b&doctype=Standard
-URIDate 2017-03-09

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- Definition

The amount of deficit or surplus in a defined benefit plan, adjusted for any effect of limiting a net defined benefit asset to the asset ceiling. [Refer: Defined benefit plans [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2017-01-01
-Paragraph 140
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_140_a&doctype=Standard
-URIDate 2017-03-09

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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from payments from the plan in respect of settlements. [Refer: Gains (losses) arising from settlements, net defined benefit liability (asset); Net defined benefit liability (asset); Defined benefit plans [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2017-01-01
-Paragraph 141
-Subparagraph g
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_g&doctype=Standard
-URIDate 2017-03-09

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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from the return on plan assets. The return on plans assets is interest, dividends and other revenue derived from the plan assets, together with realised and unrealised gains or losses on the plan assets, less any costs of managing plan assets and less any tax payable by the plan itself, other than tax included in the actuarial assumptions used to measure the present value of the defined benefit obligation. [Refer: Plan assets [member]; Net defined benefit liability (asset); Actuarial assumptions [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2017-01-01
-Paragraph 141
-Subparagraph c
-Clause i
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_c_i&doctype=Standard
-URIDate 2017-03-09

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- Definition

Administrative Expenses, Net Defined Benefit Liability (Asset)

+ References

No definition available.

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- Details

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