Income taxes - Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Loss Carryforwards [Line Items]
Withholding taxes on certain investment transactions									$ 2,573	$ 6,678	$ 4,828
Change in uncertain tax positions									155	147	(1,100)
Non-deductible expenses and other									(277)	75	0
Income tax expense (benefit)	$ (2,104)	$ 3,475	$ 5,307	$ 5,298	$ (272)	$ 2,484	$ 5,310	$ (1,929)	11,976	5,593	(2,905)
Bermuda
Operating Loss Carryforwards [Line Items]
Bermuda (expected tax expense at 0%)									0	0	0
United States
Operating Loss Carryforwards [Line Items]
Foreign location									9,510	(1,290)	(6,644)
United Kingdom
Operating Loss Carryforwards [Line Items]
Foreign location									$ 15	$ (17)	$ 11