Quarterly report pursuant to Section 13 or 15(d)

Securities Owned and Securities Sold, But Not Yet Purchased - at Fair Value (Details)

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Securities Owned and Securities Sold, But Not Yet Purchased - at Fair Value (Details) - USD ($)
$ in Thousands
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2014
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned $ 4,617 $ 4,617 $ 5,910
Securities sold, but not yet purchased, at fair value (5,183) $ (5,183) (230)
Certificates of deposit      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned 453   465
Securities sold, but not yet purchased, at fair value 0   0
Debt securities      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned 1,280   1,526
Securities sold, but not yet purchased, at fair value (233)   (45)
U.S. Treasury notes      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned 105   102
Securities sold, but not yet purchased, at fair value (147)   (151)
Common stock and warrants      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned 966   1,981
Securities sold, but not yet purchased, at fair value (4,803)   (34)
Restricted common stock and warrants      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned 950   875
Securities sold, but not yet purchased, at fair value 0   0
Other investments      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned 863   961
Securities sold, but not yet purchased, at fair value $ 0   $ 0