employee future benefits - Employer contributions (Details) - Pension plans - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
employee future benefits
Undrawn letters of credit that secure obligations	$ 173	$ 174
Estimated contributions to defined benefit plans in 2020	$ 46

The estimate of the expected contributions to be made to a defined benefit plan for the next annual reporting period. [Refer: Defined benefit plans [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2019-01-01
-Paragraph 147
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2019-en-r&anchor=para_147_b&doctype=Standard
-URIDate 2019-03-27

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2019-01-01
-Paragraph 148
-Subparagraph d
-Clause iii
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2019-en-r&anchor=para_148_d_iii&doctype=Standard
-URIDate 2019-03-27

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Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration