Securities Owned and Securities Sold, But Not Yet Purchased - at Fair Value (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	$ 5,858	$ 4,789
Securities sold, but not yet purchased	(3,434)	(83)
Certificates of deposit
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned		24
Securities sold, but not yet purchased	(66)	0
Debt securities
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	1,896	1,402
Securities sold, but not yet purchased	(66)	(46)
U.S. Treasury notes
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	1	1
Securities sold, but not yet purchased	(297)	0
Common stock and warrants
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	1,805	689
Securities sold, but not yet purchased	(3,071)	(33)
Restricted common stock and warrants
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	1,131	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	1,025	1,948
Securities sold, but not yet purchased	$ 0	$ 0