Securities Owned and Securities Sold, But Not Yet Purchased - at Fair Value (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	$ 4,986	$ 5,910
Securities sold, but not yet purchased	(321)	(230)
Certificates of deposit
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	481	465
Securities sold, but not yet purchased	(11)	0
Debt securities
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	1,962	1,526
Securities sold, but not yet purchased	(108)	(45)
U.S. Treasury notes
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	103	102
Securities sold, but not yet purchased	(201)	(151)
Common stock and warrants
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	661	1,981
Securities sold, but not yet purchased	(1)	(34)
Restricted common stock and warrants
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	871	875
Securities sold, but not yet purchased	0	0
Other investments
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	908	961
Securities sold, but not yet purchased	$ 0	$ 0