Income taxes - Schedule of net deferred tax liabilities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net deferred tax liabilities
Operating loss carry-forwards	$ 93,466	$ 61,708	$ 52,791
Pensions	1,899	2,161	1,517
Share options	2,813	1,647	0
Trademarks	5,563	3,868	0
Other	9,197	2,897	6,975
Less: Valuation allowance	(97,376)	(50,746)	(28,201)
Net deferred tax assets	15,562	21,535	33,082
Other	(5,267)	(4,609)	0
Depreciable assets	(171,016)	(172,643)	(195,647)
Deferred tax liabilities	(176,283)	(177,252)	(195,647)
Net deferred tax liabilities	$ (160,721)	$ (155,717)	$ (162,565)