Related party transactions (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	May 13, 2014	Oct. 31, 2012
Related Party Transaction [Line Items]
Operating expenses	$ 7,395,000	$ 8,585,000
Winthrop [Member]
Related Party Transaction [Line Items]
Investment management and distribution fees	$ 778,000	871,000
Lease Agreements [Member] | Bedford Oak [Member]
Related Party Transaction [Line Items]
Monthly sublease payment amount			$ 27,600	$ 40,700
Operating expenses		$ 83,000