Net investment income (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
Net Investment Income [Abstract]
Net realized gains (losses) on investments and investment derivatives									$ (5,850)	$ 446,646	$ 228,628
Net change in unrealized gains (losses) on investments and investment derivatives									8,381	(412,650)	251,496
Net gains (losses) on currencies									12,806	(7,305)	6,441
Dividend and interest income									18,551	53,523	65,896
Dividends paid on securities sold, not yet purchased									0	(5,259)	(5,724)
Other expenses									(954)	(15,696)	(24,073)
Management and performance fees to related parties (1)									0	(29,845)	(130,711)
Investment income, net, related party investment fund									249,626	(280,847)	0
Net investment income (loss)	$ 61,614	$ (3,138)	$ 69,131	$ 154,953	$ (276,810)	$ (3,590)	$ 31,175	$ (2,208)	$ 282,560	$ (251,433)	$ 391,953