Income taxes (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]
Uncertain tax positions	$ 1,500				$ 1,500				$ 1,500	$ 1,500
Income tax (expense) benefit	$ (718)	$ (213)	$ (74)	$ 1,718	$ (8,417)	$ (111)	$ 4,390	$ 128	713	(4,010)	$ 11,976
Income tax expense (benefit) related to U.S. and U.K. subsidiaries
Operating Loss Carryforwards [Line Items]
Income tax (expense) benefit									713	(10,035)	9,248
Change in uncertain tax positions
Operating Loss Carryforwards [Line Items]
Income tax (expense) benefit									0	(300)	155
Withholding taxes on certain investment transactions
Operating Loss Carryforwards [Line Items]
Income tax (expense) benefit									$ 0	$ 6,325	$ 2,573