Quarterly report pursuant to Section 13 or 15(d)

Securities Owned and Securities Sold, But Not Yet Purchased - Additional Information (Details)

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Securities Owned and Securities Sold, But Not Yet Purchased - Additional Information (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, at fair value $ 4,617 $ 4,617 $ 5,910
Securities sold, but not yet purchased, at fair value (5,183) $ (5,183) (230)
Level 1      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned, at fair value 1,419   2,446
Securities sold, but not yet purchased, at fair value (4,803)   (34)
Level 2      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned, at fair value 3,198   3,464
Securities sold, but not yet purchased, at fair value (380)   (196)
Level 3      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned, at fair value 0   0
Securities sold, but not yet purchased, at fair value 0   0
Certificates of deposit      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned, at fair value 453   465
Securities sold, but not yet purchased, at fair value 0   0
Certificates of deposit | Level 1      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned, at fair value 453   465
Securities sold, but not yet purchased, at fair value 0    
Certificates of deposit | Level 2      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned, at fair value 0   0
Securities sold, but not yet purchased, at fair value 0    
Certificates of deposit | Level 3      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned, at fair value 0   0
Securities sold, but not yet purchased, at fair value 0    
Debt securities      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned, at fair value 1,280   1,526
Securities sold, but not yet purchased, at fair value (233)   (45)
Debt securities | Level 1      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned, at fair value 0   0
Securities sold, but not yet purchased, at fair value 0   0
Debt securities | Level 2      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned, at fair value 1,280   1,526
Securities sold, but not yet purchased, at fair value (233)   (45)
Debt securities | Level 3      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned, at fair value 0   0
Securities sold, but not yet purchased, at fair value 0   0
U.S. Treasury notes      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned, at fair value 105   102
Securities sold, but not yet purchased, at fair value (147)   (151)
U.S. Treasury notes | Level 1      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned, at fair value 0   0
Securities sold, but not yet purchased, at fair value 0   0
U.S. Treasury notes | Level 2      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned, at fair value 105   102
Securities sold, but not yet purchased, at fair value (147)   (151)
U.S. Treasury notes | Level 3      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned, at fair value 0   0
Securities sold, but not yet purchased, at fair value 0   0
Common stock, restricted common stock and warrants      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned, at fair value 1,916   2,856
Securities sold, but not yet purchased, at fair value (4,803)   (34)
Common stock, restricted common stock and warrants | Level 1      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned, at fair value 966   1,981
Securities sold, but not yet purchased, at fair value (4,803)   (34)
Common stock, restricted common stock and warrants | Level 2      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned, at fair value 950   875
Securities sold, but not yet purchased, at fair value 0   0
Common stock, restricted common stock and warrants | Level 3      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned, at fair value 0   0
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, at fair value 863   961
Securities sold, but not yet purchased, at fair value 0   0
Other investments | Level 1      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned, at fair value 0   0
Other investments | Level 2      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned, at fair value 863   961
Other investments | Level 3      
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]      
Securities owned, at fair value $ 0   $ 0