Property, plant and equipment - Major classes of assets under capital leases (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Assets under capital leases
Assets under capital leases, gross	$ 864	$ 997
Less: Accumulated depreciation	(718)	(905)
Total assets under capital leases, net of accumulated depreciation	146	92
Machinery and equipment
Assets under capital leases
Assets under capital leases, gross	757	889
Fixtures, fittings and office equipment
Assets under capital leases
Assets under capital leases, gross	$ 107	$ 108