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Stock-based compensation - Director deferred compensation (Details) (Director)
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12 Months Ended | ||
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Dec. 31, 2014
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Dec. 31, 2013
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Dec. 31, 2012
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Director
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| Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] | |||
| Beginning balance, Directors deferred compensation account (in shares) | 129,264 | ||
| New shares deferred (in shares) | 13,636 | ||
| Issued (in shares) | (7,178) | (12,909) | (6,592) |
| Ending balance, Directors deferred compensation account (in shares) | 135,722 | 129,264 | |