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Share-based compensation - Narrative (Details) - USD ($) $ in Millions |  | 1 Months Ended | 3 Months Ended | 9 Months Ended
--- | --- | --- | --- | ---
Mar. 01, 2014 | Jun. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019
Share-based compensation |  |  |  |  |  |
Income tax benefit from compensation expense |  |  | $ 0.2 | $ 0.3 | $ 0.9 | $ 1.2
Restricted stock units |  |  |  |  |  |
Share-based compensation |  |  |  |  |  |
Fair value of vested stock |  |  |  |  | 4.2 | 3.2
Income tax benefit from compensation expense |  |  |  |  | 0.7 | $ 0.5
Unrecognized share based compensation |  |  | 6.2 |  | $ 6.2 |
Weighted average period for recognition of unrecognized compensation cost (in years) |  |  |  |  | 2 years 8 months 12 days |
Long-term Incentive Plan |  |  |  |  |  |
Share-based compensation |  |  |  |  |  |
Payment award, low end of range |  |  |  |  | 0.00% |
Payment award, high end of range |  |  |  |  | 200.00% |
Measurement period for total return to shareholders (in years) |  |  |  |  | 3 years |
Award performance period (in years) |  |  |  |  | 3 years |
LTIP linked to TRS |  |  |  |  |  |
Share-based compensation |  |  |  |  |  |
Fair value of vested stock |  |  |  |  | $ 2.6 |
Income tax benefit from compensation expense |  |  |  |  | 0.4 |
Unrecognized share based compensation |  |  | 1.6 |  | $ 1.6 |
Weighted average period for recognition of unrecognized compensation cost (in years) |  |  |  |  | 1 year 2 months 12 days |
LTIP awards linked to other performance conditions |  |  |  |  |  |
Share-based compensation |  |  |  |  |  |
Fair value of vested stock |  |  |  |  | $ 7.6 |
Income tax benefit from compensation expense |  |  |  |  | 1.2 |
Unrecognized share based compensation |  |  | $ 5.8 |  | $ 5.8 |
Weighted average period for recognition of unrecognized compensation cost (in years) |  |  |  |  | 1 year 4 months 24 days |
Equity and Incentive Plan |  |  |  |  |  |
Share-based compensation |  |  |  |  |  |
Number of additional shares authorized (in shares) | 1,500,000 |  |  |  |  |
Shares available for future issuance (in shares) |  |  | 3,000,000.0 |  | 3,000,000.0 |
Number of share issuable upon vesting and achievement of performance goals (in shares) |  |  | 700,000 |  | 700,000 |
Nonemployee Director Stock Plan |  |  |  |  |  |
Share-based compensation |  |  |  |  |  |
Number of additional shares authorized (in shares) |  | 300,000 |  |  |  |
Shares available for future grant (in shares) |  |  | 274,163 |  | 274,163 |

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- Definition Represents the number of shares that could be issued upon vesting of share-based payment awards and achievement of performance goals. + References No definition available. + Details
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- Definition Represents the number of authorized but unissued shares that could be issued under the equity-based compensation plan. + References No definition available. + Details
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- Definition Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents 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 (i) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Amount of tax benefit for recognition of expense of award under share-based payment arrangement. + 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 (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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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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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + 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 -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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