Income taxes (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Income taxes
Provision for income taxes	$ (5,796,000)	$ 8,762,000	$ (1,182,000)	$ 15,967,000
Increase in the beginning-of-the year balance of valuation allowances				3,827,000
Exchange rate movements on deferred tax	(4,207,000)	1,308,000	(2,867,000)	853,000
Deferred tax on derivatives	148,000	40,000	191,000	(351,000)
Changes in uncertain tax positions	(790,000)	46,000	(1,394,000)	336,000
Changes in interest and penalties	76,000	62,000	(1,076,000)	380,000
Recognized tax benefit related to prior year uncertain tax positions, interest and penalties			$ 2,471,000