Income taxes - Narratives (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Difference in taxation rates	$ 8,316	$ 13,874	$ 9,509
Change in tax rates	0	(19,807)	(643)
Non deductable tax expense	$ (190)	$ 2,330	$ 0
Statutory tax rate, percentage	19.00%	19.25%	20.00%
Gross amount of tax loss carryforwards	$ 270,615	$ 316,418
Net operating losses carryforwards expiring within five years	15,305
Net operating losses carryforwards expiring after five years but within ten years	15,459
Net operating losses carryforwards expiring after ten years or with no expiry date	239,851
Deferred tax liability, undistributed foreign earnings	1,966	1,611
Cumulative amount of unremitted earnings, foreign subsidiaries	1,388,000	1,406,000
Provision for income taxes	13,983	6,554	$ 16,368
Benefit included in tax provision, in respect of the provision for uncertain tax positions	(46)	160	3,350
Charges included in tax provision related to potential interest and penalty costs associated with uncertain tax positions	6	15	(639)
Deferred tax provision	5,547	2,215	4,876
Unrecognized tax benefit	553	532	318	$ 3,678
Unrecognized tax benefits that would affect effective tax rate if recognized	553	$ 532	$ 318
Amount by which uncertain tax position is reasonably possible to decrease within the next 12 months	$ 100