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Commitments and contingencies - Narrative (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Loss Contingencies [Line Items]          
Tax contingencies $ 2,347   $ 2,347   $ 2,310
Rental expense under non-cancelable operating leases, including contingent leases 600 $ 500 $ 1,100 $ 1,000  
Electricity          
Loss Contingencies [Line Items]          
Term of electricity supply contract     5 years    
Straight-line depreciation percent per annum     20.00%    
Capital Investment | Electricity          
Loss Contingencies [Line Items]          
Obligated refund to supplier should contract be terminated prior to end of contractual term 1,400   $ 1,400    
Unearned Rebates | Electricity          
Loss Contingencies [Line Items]          
Obligated refund to supplier should contract be terminated prior to end of contractual term 600   600   $ 800
Foreign Tax Authority | Tax and Customs Administration, Netherlands          
Loss Contingencies [Line Items]          
Tax contingencies $ 1,600   $ 1,600