Share-based payments - Narrative (Details) - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Disclosure of terms and conditions of share-based payment arrangement [line items]
Number of shares authorized (in shares)	14,586,683
ESP
Disclosure of terms and conditions of share-based payment arrangement [line items]
Maximum employee contribution rate	12.00%
Award vesting period	2 years
Number of shares authorized (in shares)	5,591,566
RSUs/PSUs
Disclosure of terms and conditions of share-based payment arrangement [line items]
Award vesting period	3 years
DSP
Disclosure of terms and conditions of share-based payment arrangement [line items]
Award vesting period	3 years
Liabilities from share-based payment transactions	$ 30	$ 37
Stock Options
Disclosure of terms and conditions of share-based payment arrangement [line items]
Award vesting period	3 years
Award exercise period	7 years
Top of range \| ESP
Disclosure of terms and conditions of share-based payment arrangement [line items]
Maximum employer contribution rate	2.00%
Bottom of range \| Deferred Stock Units (DSU)
Disclosure of terms and conditions of share-based payment arrangement [line items]
Percent of employee compensation paid in DSUs after minimum share ownership requirement	50.00%

Share-Based Compensation Arrangement By Share-Based Payment Award, Award Vesting Period2

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Share-Based Compensation Arrangement By Share-Based Payment Award, Maximum Employee Subscription Rate1

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Share-Based Compensation Arrangement Vy Share-based Payment Award, Number of Shares Authorized1

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- Definition

Share-Based Payment Arrangement, Employer Matching Contribution, Percent of Match

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Share-Based Payment Arrangement, Percent of Employee Compensation Paid in Other Equity Instruments After Minimum Share Ownership Requirement

+ References

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Share-based Compensation Arrangement By Share-based Payment Award, Exercise Period

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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

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The amount of liabilities arising from share-based payment transactions. Share-based payment transactions are transactions in which the entity: (a) receives goods or services from the supplier of those goods or services (including an employee) in a share-based payment arrangement; or (b) incurs an obligation to settle the transaction with the supplier in a share-based payment arrangement when another group entity receives those goods or services. [Refer: Share-based payment arrangements [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 2
-IssueDate 2017-01-01
-Paragraph 51
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