Securities Owned and Securities Sold, But Not Yet Purchased - Narrative (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned deposited with the Company's subsidiaries clearing broker	$ 4,003		$ 5,429
Securities owned, at fair value	4,617	$ 4,617	5,910
Common stock and warrants
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned, at fair value	966		1,981
Level 2
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned, at fair value	3,198		3,464
Level 2 \| Common stock and warrants \| Warrants to purchase common stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned, at fair value	$ 440		$ 403