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Employee benefit plan (Details) (USD $)
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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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| Employee benefit plan | |||
| Contributions made under 401(k) plan | $ 219,000 | $ 160,000 | $ 116,000 |