Related party transactions - Disclosure of transactions with key management personnel (Details) - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Related Party [Abstract]
Wages, salaries, fees and related taxes and benefits	$ (23)	$ (24)
Post-employment benefit plans and OPEBs cost	(3)	(4)
Share-based compensation	(23)	(27)
Key management personnel and board of directors compensation expense	$ (49)	$ (55)

Related Party [Abstract]

No definition available.

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- Definition

The amount of compensation to key management personnel. [Refer: Key management personnel of entity or parent [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 24
-IssueDate 2017-01-01
-Paragraph 17
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=24&date=2017-03-01&anchor=para_17&doctype=Standard
-URIDate 2017-03-09

+ Details

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- Definition

The amount of compensation to key management personnel in the form of post-employment benefits. [Refer: Key management personnel of entity or parent [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 24
-IssueDate 2017-01-01
-Paragraph 17
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=24&date=2017-03-01&anchor=para_17_b&doctype=Standard
-URIDate 2017-03-09

+ Details

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- Definition

The amount of compensation to key management personnel in the form of share-based payments. [Refer: Key management personnel of entity or parent [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 24
-IssueDate 2017-01-01
-Paragraph 17
-Subparagraph e
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=24&date=2017-03-01&anchor=para_17_e&doctype=Standard
-URIDate 2017-03-09

+ Details

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- Definition

The amount of compensation to key management personnel in the form of short-term employee benefits. [Refer: Key management personnel of entity or parent [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 24
-IssueDate 2017-01-01
-Paragraph 17
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=24&date=2017-03-01&anchor=para_17_a&doctype=Standard
-URIDate 2017-03-09

+ Details

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