Income taxes - Narratives (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax
Change in tax rates		$ (19,807)	$ (643)	$ (5,121)
Non deductable tax expense		$ 2,330	$ 0	$ 0
Statutory tax rate, percentage		19.25%	20.00%	15.00%
Gross amount of tax loss carryforwards		$ 316,418	$ 369,523
Net operating losses carryforwards expiring within five years		11,037
Net operating losses carryforwards expiring after five years but within ten years		11,272
Net operating losses carryforwards expiring after ten years or with no expiry date		294,109
Deferred tax liability, undistributed foreign earnings		1,611	1,669
Cumulative amount of unremitted earnings, foreign subsidiaries		1,406,000	1,355,000
Provision for income taxes		6,554	16,368	$ 17,041
Benefit included in tax provision, in respect of the provision for uncertain tax positions		160	(3,350)	279
Charges included in tax provision related to potential interest and penalty costs associated with uncertain tax positions		15	639	247
Deferred tax provision		2,215	4,876	16,320
Unrecognized tax benefit		532	318	3,678	$ 3,437
Unrecognized tax benefits that would affect effective tax rate if recognized		532	$ 318	$ 3,678
Amount by which uncertain tax position is reasonably possible to decrease within the next 12 months		$ 100
Foreign Tax Authority | Her Majesty's Revenue and Customs (HMRC)
Income Tax
Statutory tax rate, percentage		19.25%	20.00%
Foreign Tax Authority | Office of the Tax Commissioner, Bermuda
Income Tax
Statutory tax rate, percentage	0.00%			15.00%