Intangible assets (Details) - USD ($) $ in Thousands		Jun. 30, 2015	Dec. 31, 2014
Costs:
Patent technology		$ 4,891	$ 4,887
Management software license		940	932
Total intangible assets	[1]	5,831	5,819
Less: Amortization	[2]	(2,707)	(2,400)
Total intangible assets, net		$ 3,124	$ 3,419

[1] Through the acquisition of Fujian Qiaolong in the second quarter of 2014, the Company acquired $0.9 million of intangible assets, consisting of $0.9 million of patent technology and $nil of management software licenses, which are amortized over a weighted average life of 4.7 years and 1.1 years, respectively.

[2] Amortization expenses were $0.30 million and $0.12 million for the three months ended June 30, 2015 and 2014, respectively, and $0.5 million and $0.17 million for the six months ended June 30, 2015 and 2014, respectively.