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Share-based compensation - Restricted stock units - Summary of awards (Details) - Restricted stock units - $ / shares								9 Months Ended
	May 29, 2021	Mar. 04, 2021	May 29, 2020	Mar. 04, 2020	May 28, 2019	Mar. 07, 2019	Jan. 02, 2019	Sep. 30, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
SARs Awarded (in shares)								297,136
Service Period One Year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
SARs Awarded (in shares)	95,583	56,957	78,510		59,237
Service Period	1 year	1 year	1 year		1 year
Grant Price (in dollars per share)	$ 4.31	$ 4.28	$ 5.84		$ 7.47
Service Period Two Years
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
SARs Awarded (in shares)							176,659
Service Period							2 years
Grant Price (in dollars per share)							$ 4.64
Service Period Three Years
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
SARs Awarded (in shares)		144,596		94,105		86,210
Service Period		3 years		3 years		3 years
Grant Price (in dollars per share)		$ 4.28		$ 5.25		$ 5.10

X

- Definition

Granted price per share of equity-based compensation awards granted. Excludes stock and unit options.

+ References

No definition available.

+ Details

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X

- Definition

Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

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- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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