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Stock based compensation (Details) (USD $) In Thousands, except Share data, unless otherwise specified | 12 Months Ended |  | 0 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009 | Dec. 31, 2011 Stock Appreciation Rights (SARs) [Member] | Dec. 31, 2011 Restricted Stock Units (RSUs) [Member] | May 26, 2011 Deferred Stock Units [Member] | Dec. 31, 2011 Deferred Stock Units [Member] | Dec. 31, 2011 Performance Shares [Member]
Stock-Based Compensation [Abstract] |  |  |  |  |  |  |  |
Shares available for future grant under the plan | 893,952 |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number | 0 |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award Non Option Equity Instruments Granted Per Director |  |  |  |  |  | 131 |  |
Expense recognized in connection with the issuance of Deferred Stock Units | $ 300 | $ 0 | $ 0 |  |  |  | $ 300 |
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Outstanding Number |  |  |  | 0 | 0 |  | 1,048 | 0

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- Definition The noncash expense that represents the cost of restricted stock or unit distributed to employees as compensation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585 + 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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