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Share-based compensation - Narrative (Details) - USD ($) $ in Millions |  | 3 Months Ended | 6 Months Ended
--- | --- | --- | ---
Mar. 01, 2014 | Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017
Share-based compensation |  |  |  |  |
Income tax benefit from compensation expense |  | $ 0.5 | $ 0.8 | $ 0.7 | $ 1.2
Restricted stock units |  |  |  |  |
Share-based compensation |  |  |  |  |
Fair value of vested stock |  |  |  | 2.7 | 3.3
Income tax benefit from compensation expense |  |  |  | 0.4 | 1.2
Unrecognized share based compensation |  | 5.4 |  | $ 5.4 |
Weighted average period for recognition of unrecognized compensation cost (in years) |  |  |  | 2 years 9 months 18 days |
Long-term Incentive Plan |  |  |  |  |
Share-based compensation |  |  |  |  |
Payment award, low end of range |  |  |  | 0.00% |
Payment award, high end of range |  |  |  | 200.00% |
Award performance period (in years) |  |  |  | 3 years |
LTIP linked to TRS |  |  |  |  |
Share-based compensation |  |  |  |  |
Fair value of vested stock |  |  |  |  | 1.9
Unrecognized share based compensation |  | 1.7 |  | $ 1.7 |
Weighted average period for recognition of unrecognized compensation cost (in years) |  |  |  | 2 years |
Measurement period for total return to shareholders (in years) |  |  |  | 3 years |
Tax benefits related to awards vested |  |  |  |  | 0.7
LTIP awards linked to other performance conditions |  |  |  |  |
Share-based compensation |  |  |  |  |
Fair value of vested stock |  |  |  |  | 4.2
Income tax benefit from compensation expense |  |  |  |  | $ 1.6
Unrecognized share based compensation |  | $ 6.0 |  | $ 6.0 |
Weighted average period for recognition of unrecognized compensation cost (in years) |  |  |  | 2 years |
Equity and Incentive Plan |  |  |  |  |
Share-based compensation |  |  |  |  |
Number of additional shares authorized (in shares) | 1,500,000 |  |  |  |
Shares available for future issuance (in shares) |  | 3,200,000 |  | 3,200,000 |
Number of share issuable upon vesting and achievement of performance goals (in shares) |  | 600,000 |  | 600,000 |
Nonemployee Director Stock Plan |  |  |  |  |
Share-based compensation |  |  |  |  |
Shares available for future grant (in shares) |  | 46,607 |  | 46,607 |

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- Definition Represents the number of years used in the calculation of total return to shareholders. + References No definition available. + Details
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- Definition Represents the high end of range of payout depending on achievement of the goals. + References No definition available. + Details
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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 Share-based Compensation Arrangement by Share-based Payment Award, Award Performance Period + References No definition available. + Details
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- Definition Unrecognized cost of unvested share-based compensation awards. + 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=109197908&loc=d3e5070-113901 + Details
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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the 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=109197908&loc=d3e5070-113901 + Details
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- Definition The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the 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 (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition Amount of tax benefit from exercise of stock options granted under share-based compensation 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 2A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109197908&loc=SL79508275-113901 + 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=109197908&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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- Definition Number of additional shares authorized for issuance under an established share-based compensation plan. + References No definition available. + 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=109197908&loc=d3e5070-113901 + Details
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