Equity-based compensation Plan (Equity-Based Compensation Plan) (Details) - $ / shares	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding RollForward
Number of options - Beginning of period	917,269	951,523	888,571
Number of options - Options granted	0	0	179,225
Number of options - Options exercises	71,150	24,752	112,704
Number of options - Options forfeited	500	9,502	3,569
Number of options - End of period	845,619	917,269	951,523
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price Rollforward
Weighted Average Exercise Price - Beginning of period	$ 14.08	$ 12.45	$ 14.12
Weighted Average Exercise Price - Options granted	0	0	17.44
Weighted Average Exercise Price - Options exercises	12.96	12.43	19.78
Weighted Average Exercise Price - Options forfeited	15.23	16.68	16.06
Weighted Average Exercise Price - End of period	$ 14.14	$ 14.08	$ 12.45

The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(3)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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Gross number of share options (or share units) granted during the period.

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

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Number of share options (or share units) exercised during the current period.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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