Financial and capital management - Change in Allowance for Doubtful Accounts (Details) - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Trade receivables
Reconciliation of changes in allowance account for credit losses of financial assets [abstract]
Balance, January 1	$ (54)	$ (60)
Additions	(84)	(99)
Usage	91	105
Balance, December 31	(51)	(54)
Trade receivables \| IFRS 9
Reconciliation of changes in allowance account for credit losses of financial assets [abstract]
Balance, January 1	(4)	0
Balance, December 31		(4)
Contract assets
Reconciliation of changes in allowance account for credit losses of financial assets [abstract]
Balance, January 1	(96)	(92)
Additions	(50)	(39)
Usage	55	35
Balance, December 31	(91)	(96)
Current contract assets
Reconciliation of changes in allowance account for credit losses of financial assets [abstract]
Balance, January 1	(47)
Balance, December 31	(44)	(47)
Noncurrent contract assets
Reconciliation of changes in allowance account for credit losses of financial assets [abstract]
Balance, January 1	(49)
Balance, December 31	$ (47)	$ (49)

The amount of additional allowance for credit losses of financial assets recognised in profit or loss. [Refer: Allowance account for credit losses of financial assets]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Note Expiry date 2021-01-01
-Name IFRS
-Number 7
-IssueDate 2018-01-01
-Paragraph 16
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-02&anchor=para_16&doctype=Standard&book=b
-URIDate 2018-03-16

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of an allowance account used to record impairments to financial assets due to credit losses. [Refer: Financial assets]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Note Expiry date 2021-01-01
-Name IFRS
-Number 7
-IssueDate 2018-01-01
-Paragraph 16
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-02&anchor=para_16&doctype=Standard&book=b
-URIDate 2018-03-16

+ Details

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- References

No definition available.

+ Details

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- Definition

The decrease in an allowance account for credit losses of financial assets resulting from the utilisation of the allowance. [Refer: Allowance account for credit losses of financial assets]

+ References

+ Details

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Balance Type:	debit
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- Details

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- Details

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