Income taxes - Schedule of refund of AMT carryforward (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Income Tax Disclosure [Abstract]
2019	$ 2,513
2020	1,257
2021	628
2022	628
AMT credit carryforward	$ 5,026

Deferred Tax Asset, Alternative Minimum Tax Carryforward Refund Receivable, In Year 3

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Deferred Tax Asset, Alternative Minimum Tax Carryforward Refund Receivable, Next Two Years

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Deferred Tax Asset, Alternative Minimum Tax Carryforward Refund Receivable, Total

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Deferred Tax Asset, Alternative Minimum Tax Carryforward Refund Receivable, Year 4

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Deferred Tax Asset, Alternative Minimum Tax Carryforward Refund Receivable, Year 5

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