Income taxes - Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Withholding taxes related to dividend and interest income									$ 0	$ 6,325	$ 2,573
Uncertain tax positions									0	(300)	155
Non-deductible expenses and other									10	(1)	(277)
Income tax expense (benefit)	$ (718)	$ (213)	$ (74)	$ 1,718	$ (8,417)	$ (111)	$ 4,390	$ 128	713	(4,010)	11,976
Bermuda
Operating Loss Carryforwards [Line Items]
Bermuda (expected tax expense at 0%)									0	0	0
United States
Operating Loss Carryforwards [Line Items]
Foreign location									681	(10,032)	9,510
United Kingdom
Operating Loss Carryforwards [Line Items]
Foreign location									$ 22	$ (2)	$ 15