Non-controlling interests (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Noncontrolling Interest [Line Items]
Balance at beginning of the period				$ 8,912	$ 45,071	$ 45,071
Fair value of the non-controlling interests arising from the acquisition of Fujian Qiaolong	[1]			0	2,793	2,793
Acquisition of the non-controlling interests in Henglong and Jiulong (See Note 19)				$ 0	(29,812)	(29,812)
Other comprehensive income related to the non-controlling interests acquired by the Company (See Note 19)					(4,743)	(4,743)
Income attributable to non-controlling interests		$ 93	$ 1,293	$ (56)	5,409	6,052
Dividends declared to the non-controlling interest holders of joint-venture companies (See Note 16)				(318)	(10,077)	(10,077)
Foreign currency translation adjustment attributable to non-controlling interests				(593)	(403)	(372)
Balance at end of the period		$ 7,945	$ 8,238	$ 7,945	$ 8,238	$ 8,912

[1] In the second quarter of 2014, the Company acquired a 51.0% equity interest in Fujian Qiaolong (See Note 3). The fair value of the non-controlling interest of Fujian Qiaolong is $2.8 million.