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Share-based compensation - The 2011 Director Plan (Details) - USD ($) $ in Millions | 12 Months Ended
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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

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- Definition 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 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 (d)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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