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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 |  |

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- 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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- 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 + Details
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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 + Details
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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 No definition available. + Details
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