Income taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Advance customer deposits	$ 6,010	$ 6,557
Trade payables and other accruals	5,072	4,531
Labor liability accrual	688	555
Property, plant and equipment	45	12
Net operating losses	101,003	82,356
Total deferred tax asset	112,818	94,011
Valuation allowance	(98,755)	(81,738)
Net deferred tax asset	14,063	12,273
Deferred tax liabilities:
Accounts receivable and prepayments to vendors	928	617
Property, plant and equipment	89,080	86,620
Insurance recoverable	73	50
Total deferred tax liability	90,081	87,287
Net deferred tax liability	$ (76,018)	$ (75,014)