Income tax expense - Deferred Income Tax (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 108,095	$ 94,014
Tax loss on liquidation of subsidiary	4,029	5,024
Equity compensation	4,449	4,182
Non-deductible reserves	447	395
Total deferred tax assets	117,020	103,615
Valuation allowance	(116,200)	(103,185)	$ (100,832)	$ (80,087)
Net deferred tax assets	820	430
Deferred tax liabilities:
Financial liabilities	757	48
Property, plant and equipment	63	382
Total deferred tax liability	820	430
Deferred tax, net	$ 0	$ 0