Share-based compensation - The 2011 Director Plan (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Income tax benefit	$ 1.4	$ 1.0	$ 1.4
Common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)	29,816	36,100	36,344
Fair value	$ 1.3	$ 1.3	$ 1.6
Income tax benefit	$ 0.3	$ 0.3	$ 0.4

Amount of tax benefit for recognition of expense of award under share-based payment arrangement.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

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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.

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Number of shares issued under share-based payment arrangement.

+ References

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-Name Accounting Standards Codification
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