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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

X

- 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

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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

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

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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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