Income taxes (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Tax Disclosure [Abstract]
Deferred tax charges (credit) in respect of foreign exchange gain/loss	$ (3,207,000)	$ 83,000	$ (2,094,000)
Operating loss carry-forwards	122,500,000	93,466,000	61,708,000
Gross amount of tax loss carryforwards	429,461,000	350,926,000
Net operating losses carryforwards expiring within five years	10,742,000
Net operating losses carryforwards expiring after five years but within ten years	121,144,000
Net operating losses carryforwards expiring after ten years or with no expiry date	297,575,000
Deferred Tax Liabilities, Undistributed Foreign Earnings	1,509,000	0
Retained Earnings, Undistributed Earnings from Equity Method Investees	970,000,000	1,075,000,000
Income tax uncertainties
Provision for income taxes	17,628,000	21,651,000	19,611,000
Benefit included in tax provision, in respect of the provision for uncertain tax positions	(1,593,000)	(174,000)	(3,439,000)
Charge (benefit) included in tax provision related to potential interest and penalty costs associated with uncertain tax positions	(1,654,000)	121,000	(2,058,000)
Change in valuation allowance	13,015,000	6,093,000	11,795,000
Unrecognized tax benefit	2,988,000	4,581,000	4,755,000	8,194,000
Unrecognized tax benefit, if recognized, would affect the effective tax rate	2,054,000	4,581,000	4,755,000
Amount by which uncertain tax position is reasonably possible to decrease within the next 12 months	$ (20,000)