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Share-based compensation - Additional Information (Details) $ in Millions |  | 1 Months Ended | 12 Months Ended
--- | --- | --- | ---
Mar. 01, 2014 shares | Jul. 31, 2016 USD ($) | Dec. 31, 2017 USD ($) increment shares | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Share-based compensation |  |  |  |  |
Share-based compensation expense (in dollars) |  |  | $ 5.4 | $ 4.8 | $ 6.5
Income tax benefit |  |  | $ 1.9 | 1.6 | 2.3
Long-term incentive plan (LTIP) |  |  |  |  |
Share-based compensation |  |  |  |  |
Performance period |  |  | 3 years |  |
Exception to forfeiture, minimum requisite service period |  |  | 12 months |  |
Unrecognized compensation cost |  |  | $ 0.9 |  |
Payout low end of range |  |  | 0.00% |  |
Payout high end of range |  |  | 200.00% |  |
Intrinsic value of shares exercised |  |  | $ 1.9 |  |
Tax benefit realized for the deduction of exercises |  |  | $ 0.7 |  |
LTIP linked to TRS |  |  |  |  |
Share-based compensation |  |  |  |  |
Weighted-average period over which unrecognized compensation cost expected to be recognized |  |  | 2 years |  |
Restricted stock units |  |  |  |  |
Share-based compensation |  |  |  |  |
Fair value of vested stock |  |  | $ 3.5 | 2.8 | 3.7
Income tax benefit |  |  | 1.1 | 0.9 | 1.1
Unrecognized compensation cost |  |  | $ 4.0 |  |
Weighted-average period over which unrecognized compensation cost expected to be recognized |  |  | 2 years 4 months 24 days |  |
LTIP awards linked to other performance conditions |  |  |  |  |
Share-based compensation |  |  |  |  |
Fair value of vested stock |  |  | $ 4.2 | 3.6 | 4.7
Income tax benefit |  |  | 1.6 | $ 1.4 | $ 1.8
Unrecognized compensation cost |  |  | $ 2.9 |  |
Weighted-average period over which unrecognized compensation cost expected to be recognized |  |  | 2 years |  |
Equity and Incentive Plan (EIP) |  |  |  |  |
Share-based compensation |  |  |  |  |
Additional shares available for issuance | shares | 1,500,000 |  |  |  |
Shares remaining available for future issuance | shares |  |  | 3,300,000 |  |
Shares that can be issued upon vesting of outstanding units and achievement of performance goals | shares |  |  | 400,000 |  |
Equity and Incentive Plan (EIP) | Restricted shares |  |  |  |  |
Share-based compensation |  |  |  |  |
The number of equal annual increments for which the awards become unrestricted (in installments) | increment |  |  | 4 |  |
Nonemployee Director Stock Plan |  |  |  |  |
Share-based compensation |  |  |  |  |
Shares remaining available for future issuance | shares |  |  | 85,428 |  |
Nonemployee Director Stock Plan | Stock Compensation Plan |  |  |  |  |
Share-based compensation |  |  |  |  |
Share-based compensation expense (in dollars) |  | $ 0.2 |  |  |

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- Definition Disclosure of the aggregate tax benefit realized from the exercise of stocks and the conversion of similar instruments during the annual period. + 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 expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -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=6415400&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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=6415400&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://www.xbrl.org/2003/role/presentationRef -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=6415400&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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=6415400&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 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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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