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Share-based compensation - Additional Information (Details) - shares |  |  |  |  |  |  |  |  |  | 1 Months Ended | 9 Months Ended
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 04, 2021 | Mar. 04, 2020 | Nov. 04, 2019 | Mar. 07, 2019 | Apr. 04, 2018 | Jan. 15, 2016 | Mar. 06, 2015 | Sep. 01, 2014 | Mar. 12, 2014 | Apr. 30, 2021 | Sep. 30, 2021
Stock appreciation rights |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |
RSU granted (in shares) | 610,691 | 549,020 |  | 560,000 | 1,719,733 | 205,519 | 37,797 | 5,595 | 22,118 |  |
Stock appreciation rights | Equity Incentive Plan 2013 [Member] | Officers And Directors [Member] |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |
RSU granted (in shares) |  |  |  |  |  |  |  |  |  |  | 3,710,473
RSUs forfeited (in shares) |  |  |  |  |  |  |  |  |  |  | 5,779
Dividend equivalent rights |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |
RSU granted (in shares) |  |  | 1,146,517 |  |  |  |  |  |  |  | 0
RSUs forfeited (in shares) |  |  |  |  |  |  |  |  |  |  | 0
Dividend equivalent rights | Equity Incentive Plan 2013 [Member] | Officers And Directors [Member] |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |
RSU granted (in shares) |  |  |  |  |  |  |  |  |  |  | 1,146,517
Restricted stock units |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |
RSU granted (in shares) |  |  |  |  |  |  |  |  |  |  | 297,136
RSU's vested (in shares) |  |  |  |  |  |  |  |  |  |  | 226,945
Restricted stock units | Equity Incentive Plan 2013 [Member] | Officers And Directors [Member] |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |
RSU granted (in shares) |  |  |  |  |  |  |  |  |  |  | 791,857
RSU's vested (in shares) |  |  |  |  |  |  |  |  |  | 403,249 |

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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period. + 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)(1)(iv)(3) -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 The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period. + 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)(2) -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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