Share-based compensation (Details) $ in Thousands	12 Months Ended
Dec. 31, 2017 USD ($) award_type shares	Dec. 31, 2016 USD ($) shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding, fully vested and exercisable (shares)	166,757
Options to purchase common stock (shares)		359,570
Service-Based Share Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service-based share awards outstanding (shares)	845,408	741,036
Amended and Restated Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized (shares)	10,000,000
Shares remaining (shares)	6,273,000
Share based compensation, vesting rights (in time period)	3 years
Expected term (years)	10 years
Amended and Restated Stock Option Plan [Member] \| Non-employee director [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation, vesting rights (in time period)	12 months
Amended and Restated Stock Option Plan [Member] \| Service-Based Share Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service-based share awards outstanding (shares)	845,408
Amended and Restated Stock Option Plan [Member] \| Initial vesting date [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting rights	33.33%
Amended and Restated Stock Option Plan [Member] \| Year one [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting rights	33.33%
Amended and Restated Stock Option Plan [Member] \| Year two [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting rights	33.33%
Long Term Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation, vesting rights (in time period)	3 years
Number of award types provided \| award_type	2
Performance unit liability \| $	$ 7,481	$ 5,511

Share-based Compensation Arrangement by Share-based Payment Award, Number of Award Types

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- Definition

Aggregate carrying value as of the balance sheet date of the liabilities for all deferred compensation arrangements. Represents currently earned compensation under compensation arrangements that is not actually paid until a later date.

+ References

No definition available.

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- Definition

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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- Definition

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

+ References

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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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No definition available.

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The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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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.

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- Definition

The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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- Definition

Number of options outstanding, including both vested and non-vested options.

+ 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=96867065&loc=d3e5070-113901

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-Name Accounting Standards Codification
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-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

Percentage of vesting of share-based compensation awards.

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- Definition

Expected term of share-based compensation awards, 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
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