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
Sep. 30, 2015
Dec. 31, 2014
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]    
Securities owned, at fair value $ 4,125 $ 5,910
Securities sold, but not yet purchased, at fair value (419) (230)
Level 1    
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]    
Securities owned, at fair value 1,139 2,446
Securities sold, but not yet purchased, at fair value (56) (34)
Level 2    
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]    
Securities owned, at fair value 2,986 3,464
Securities sold, but not yet purchased, at fair value (363) (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 423 465
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 423 465
Certificates of deposit | Level 2    
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]    
Securities owned, at fair value 0 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
Debt securities    
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]    
Securities owned, at fair value 1,351 1,526
Securities sold, but not yet purchased, at fair value (15) (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 1,351 1,526
Securities sold, but not yet purchased, at fair value (15) (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 113 102
Securities sold, but not yet purchased, at fair value (348) (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 113 102
Securities sold, but not yet purchased, at fair value (348) (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 and warrants    
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]    
Securities owned, at fair value 1,558 2,856
Securities sold, but not yet purchased, at fair value (56) (34)
Common stock and warrants | Level 1    
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]    
Securities owned, at fair value 716 1,981
Securities sold, but not yet purchased, at fair value (56) (34)
Common stock and warrants | Level 2    
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]    
Securities owned, at fair value 842 875
Securities sold, but not yet purchased, at fair value 0 0
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 680 961
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 680 961
Other investments | Level 3    
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]    
Securities owned, at fair value $ 0 $ 0