Property, plant and equipment - Major classes of assets under capital leases (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Assets under capital leases
Assets under capital leases, gross	$ 866	$ 897
Less: Accumulated depreciation	(656)	(652)
Total assets under capital leases, net of accumulated depreciation	210	245
Machinery and equipment
Assets under capital leases
Assets under capital leases, gross	695	716
Fixtures, fittings and office equipment
Assets under capital leases
Assets under capital leases, gross	$ 171	$ 181