Securities Owned and Sold, Not Yet Purchased (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Securities owned and sold, not yet purchased
Securities owned, at fair value	$ 1,747	$ 2,557
Securities sold, not yet purchased, at fair value	42	249
Corporate stocks-trading securities
Securities owned and sold, not yet purchased
Securities owned, at fair value	44	244
Securities sold, not yet purchased, at fair value	42	249
Mutual funds
Securities owned and sold, not yet purchased
Securities owned, at fair value	$ 1,703	$ 2,313