Income taxes (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
	Apr. 01, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax [Line Items]
Difference in taxation rates			$ 9,509	$ 12,948	$ 15,089
Provision for income taxes			16,368	17,041	$ 15,542
Gross amount of tax loss carryforwards			369,523	$ 354,717
Net operating losses carryforwards expiring within five years			11,715
Net operating losses carryforwards expiring after five years but within ten years			78,755
Net operating losses carryforwards expiring after ten years or with no expiry date			$ 279,053
Statutory tax rate, percentage			20.00%	15.00%	0.00%
Deferred tax liability, undistributed foreign earnings			$ 1,669	$ 541
Cumulative amount of unremitted earnings, foreign subsidiaries			1,355,000	1,267,000
Benefit included in tax provision, in respect of the provision for uncertain tax positions			(3,350)	279	$ 545
Charge (benefit) included in tax provision related to potential interest and penalty costs associated with uncertain tax positions			639	247	(426)
Deferred tax provision			4,876	16,320	4,573
Unrecognized tax benefit			318	3,678	3,437	$ 2,988
Unrecognized tax benefits that would affect effective tax rate if recognized			318	$ 3,678	$ 3,437
Amount by which uncertain tax position is reasonably possible to decrease within the next 12 months			$ 80
Foreign Tax Authority | Her Majesty's Revenue and Customs (HMRC)
Income Tax [Line Items]
Statutory tax rate, percentage	20.00%
Foreign Tax Authority | Office of the Tax Commissioner, Bermuda
Income Tax [Line Items]
Statutory tax rate, percentage		0.00%