Securities Owned and Sold, Not Yet Purchased (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Securities owned and sold, not yet purchased
Securities owned, at fair value	$ 1,256	$ 1,559
Securities sold, not yet purchased, at fair value		1
Corporate stocks-trading securities
Securities owned and sold, not yet purchased
Securities owned, at fair value	14	78
Securities sold, not yet purchased, at fair value		1
Mutual funds
Securities owned and sold, not yet purchased
Securities owned, at fair value	$ 1,242	$ 1,481