Net investment income - TP Fund income Statement (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Investment loss
Total investment income	$ (3,590,000)	$ 88,968,000	$ 25,377,000	$ 324,803,000
Expenses
Other expenses			(6,616,000)	(8,852,000)
Net income (loss)	(13,277,000)	$ 55,644,000	$ (19,457,000)	$ 236,609,000
TP Fund [Member]
Investment loss
Net realized loss from securities, derivative contracts and foreign currency translations	(4,113,000)
Net change in unrealized gain on securities, derivative contracts and foreign currency translations	3,654,000
Net income from currencies	2,606,000
Dividend and interest income	3,818,000
Total investment income	5,965,000
Expenses
Management fees	(2,834,000)
Other expenses	(5,685,000)
Total expenses	(8,519,000)
Net income (loss)	$ (2,554,000)