Pension and other post-retirement benefits - Defined benefit plans (Details) - CAD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Actuarial loss (gain) arising from:
Administration	$ 145,344	$ 124,869
Pension plans [member]
Disclosure of benefit plans [line items]
Current service cost	1,977	1,586
Interest income or cost	(1,476)	(1,545)
Return on assets excluding interest income	(130)	(280)
Pension plans [member] \| Plan assets [member]
Disclosure of benefit plans [line items]
Fair value of plan assets, beginning of year	6,806	7,177
Interest income or cost	197	262
Return on assets excluding interest income	130	280
Employer contributions	0	0
Actuarial loss (gain) arising from:
Benefits paid	(915)	(912)
Administration	(1)	(1)
Fair value of plan assets, end of year	6,217	6,806
Pension plans [member] \| Defined benefit obligations [member]
Disclosure of benefit plans [line items]
Defined benefit obligation, beginning of year	62,588	54,271
Current service cost	1,977	1,586
Interest income or cost	1,673	1,807
Actuarial loss (gain) arising from:
- demographic assumptions	0	0
- financial assumptions	6,323	6,925
- experience adjustment	350	777
Past service cost	0	0
Benefits paid	(1,765)	(1,705)
Foreign exchange	973	(1,073)
Defined benefit obligation, end of year	72,119	62,588
Defined benefit liability	(65,902)	(55,782)
Other Benefit Plans [Member]
Disclosure of benefit plans [line items]
Current service cost	1,010	817
Interest income or cost	(792)	(841)
Return on assets excluding interest income	0	0
Actuarial loss (gain) arising from:
Past service cost	0	0
Other Benefit Plans [Member] \| Defined benefit obligations [member]
Disclosure of benefit plans [line items]
Defined benefit obligation, beginning of year	24,955	21,161
Current service cost	1,010	817
Interest income or cost	792	841
Actuarial loss (gain) arising from:
- demographic assumptions	102	0
- financial assumptions	2,013	2,877
- experience adjustment	(2,236)	114
Past service cost	0	0
Benefits paid	(809)	(855)
Foreign exchange	0	0
Defined benefit obligation, end of year	25,827	24,955
Defined benefit liability	$ (25,827)	$ (24,955)

The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from changes in demographic assumptions that result in remeasurements of the net defined benefit liability (asset). Demographic assumptions deal with matters such as: (a) mortality; (b) rates of employee turnover, disability and early retirement; (c) the proportion of plan members with dependants who will be eligible for benefits; (d) the proportion of plan members who will select each form of payment option available under the plan terms; and (e) claim rates under medical plans. [Refer: Net defined benefit liability (asset)]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2020-01-01
-Paragraph 141
-Subparagraph c
-Clause ii
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_141_c_ii&doctype=Standard
-URIDate 2020-01-01