Securities Owned and Securities Sold, But Not Yet Purchased - at Fair Value (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	$ 6,717	$ 4,789
Securities sold, but not yet purchased	(6,010)	(83)
Debt securities
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	1,257	1,402
Securities sold, but not yet purchased	(15)	(46)
Certificates of deposit
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	113	24
Securities sold, but not yet purchased	(21)	0
U.S. Treasury notes
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	4	1
Securities sold, but not yet purchased	(50)	0
Common stock and warrants
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	3,195	689
Securities sold, but not yet purchased	(5,924)	(33)
Restricted common stock and warrants
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	862	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	1,286	1,948
Securities sold, but not yet purchased	$ 0	$ 0