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Employee benefit plan - Additional Information (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Retirement Benefits [Abstract] |  |  |
Defined contribution plan, vest percentage | 100.00% |  |
Company contributions to 401 (k) plan | $ 0.2 | $ 0.2 | $ 0.1

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- References No definition available. + Details
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- Definition Amount of discretionary contributions made by an employer to a defined contribution plan. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
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- Definition Percentage of employer's matching contributions to a defined contribution plan that vests in a given year. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
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