Securities Owned and Securities Sold, But Not Yet Purchased - at Fair Value (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	$ 4,487	$ 4,789
Securities sold, but not yet purchased	(353)	(83)
Debt securities
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	1,491	1,402
Securities sold, but not yet purchased	(99)	(46)
Certificates of deposit
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	3	24
Securities sold, but not yet purchased	0	0
U.S. Treasury notes
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	3	1
Securities sold, but not yet purchased	(200)	0
Common stock and warrants
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	1,019	689
Securities sold, but not yet purchased	(54)	(33)
Restricted common stock and warrants
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	922	725
Securities sold, but not yet purchased	0	(4)
Other investments
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	1,049	1,948
Securities sold, but not yet purchased	$ 0	$ 0