Share-based compensation plans - Inputs used in the measurement - Cash (Details)	12 Months Ended
Dec. 31, 2020 CAD ($) shares yr $ / shares	Dec. 31, 2019 shares $ / shares	Dec. 31, 2018 shares
Disclosure of terms and conditions of share-based payment arrangement [line items]
Number of options granted \| shares	0	886,740
Average strike price	$ 0	$ 15.27
Number of options, reporting date \| shares	6,158,539	8,617,097	8,820,805
Average strike price, reporting date	$ 17.73	$ 18.9
Performance Share Unit Plan [member]
Disclosure of terms and conditions of share-based payment arrangement [line items]
Number of options granted \| shares	636,570
Expected vesting	102.00%
Average strike price	$ 0
Expected dividend \| $	$ 0
Expected volatility	0.00%
Risk-free interest rate	0.00%
Expected life of option \| yr	3
Expected forfeitures	12.00%
Weighted average grant date fair values	$ 11.45
Number of options, reporting date \| shares	1,720,636
Expected vesting, reporting date	129.00%
Expected dividend, reporting date \| $	$ 0
Expected volatility, reporting date	22.00%
Risk free interest rate, reporting date	0.10%
Expected life of option, reporting date \| yr	1
Expected forfeitures, reporting date	11.00%
Weighted average share price, reporting date	$ 22.01
Restricted Share Unit Plan [member]
Disclosure of terms and conditions of share-based payment arrangement [line items]
Number of options granted \| shares	423,180
Expected vesting	0.00%
Average strike price	$ 0
Expected dividend \| $	$ 0
Expected volatility	0.00%
Risk-free interest rate	0.00%
Expected life of option \| yr	3
Expected forfeitures	12.00%
Weighted average grant date fair values	$ 11.45
Number of options, reporting date \| shares	425,176
Expected vesting, reporting date	0.00%
Expected dividend, reporting date \| $	$ 0
Expected volatility, reporting date	0.00%
Risk free interest rate, reporting date	0.00%
Expected life of option, reporting date \| yr	2.2
Expected forfeitures, reporting date	12.00%
Weighted average share price, reporting date	$ 17.05
Phantom Stock Option Plan [member]
Disclosure of terms and conditions of share-based payment arrangement [line items]
Number of options granted \| shares	112,140
Expected vesting	0.00%
Average strike price	$ 11.61
Expected dividend \| $	$ 0.08
Expected volatility	39.00%
Risk-free interest rate	0.60%
Expected life of option \| yr	4
Expected forfeitures	8.00%
Weighted average grant date fair values	$ 2.85
Number of options, reporting date \| shares	422,291
Expected vesting, reporting date	0.00%
Average strike price, reporting date	$ 15.66	$ 16.48
Expected dividend, reporting date \| $	$ 0.08
Expected volatility, reporting date	41.00%
Risk free interest rate, reporting date	0.30%
Expected life of option, reporting date \| yr	3.4
Expected forfeitures, reporting date	7.00%
Weighted average share price, reporting date	$ 5.43

The expected forfeitures of the share price used to calculate the fair value of the share options granted.

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Description of expeted forfeitures share options oustanding.

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- Definition

The amount of expected vesting used to calculate the fair value of share options.

+ References

No definition available.

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- Definition

The option life of share options oustanding.

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No definition available.

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- Definition

The amount of an expected dividend used to calculate the fair value of share options.

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- Definition

The expected volatility of the share price used to calculate the fair value of the share options on the reporting date.

+ References

No definition available.

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- Definition

The expected vesting used to calculate the fair value of the share options on the reporting date.

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No definition available.

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- Definition

The implied yield currently available on zero-coupon government issues of the country in whose currency the exercise price for share options outstanding expressed, with a remaining term equal to the expected term of the option being valued (based on the option's remaining contractual life and taking into account the effects of expected early exercise).

+ References

No definition available.

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- Definition

The expected volatility of the share price used to calculate the fair value of the share options granted. Expected volatility is a measure of the amount by which a price is expected to fluctuate during a period. The measure of volatility used in option pricing models is the annualised standard deviation of the continuously compounded rates of return on the share over a period of time.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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The option life of share options granted.

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The implied yield currently available on zero-coupon government issues of the country in whose currency the exercise price for share options granted is expressed, with a remaining term equal to the expected term of the option being valued (based on the option's remaining contractual life and taking into account the effects of expected early exercise). [Refer: Government [member]]

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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.

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No definition available.

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The amount of an expected dividend used to calculate the fair value of share options granted.

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The number of share options outstanding in a share-based payment arrangement.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 2
-IssueDate 2020-01-01
-Paragraph 45
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
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The number of share options granted in a share-based payment arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 2
-IssueDate 2020-01-01
-Paragraph 45
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- Definition

The weighted average exercise price of share options exercisable in a share-based payment arrangement. [Refer: Weighted average [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 2
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- Definition

The weighted average exercise price of share options granted in a share-based payment arrangement. [Refer: Weighted average [member]]

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The weighted average share price. [Refer: Weighted average [member]]

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

The weighted average share price used as input to the option pricing model to calculate the fair value of share options granted. [Refer: Option pricing model [member]; Weighted average [member]]

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