Quarterly report pursuant to Section 13 or 15(d)

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

Securities Owned and Securities Sold, But Not Yet Purchased - Additional Information (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned at fair value	$ 4,986	$ 5,910
Securities sold, but not yet purchased	(321)	(230)
Level 1
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned at fair value	1,142	2,446
Securities sold, but not yet purchased	(12)	(34)
Level 2
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned at fair value	3,844	3,464
Securities sold, but not yet purchased	(309)	(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	0	0
Certificates of deposit
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned at fair value	481	465
Securities sold, but not yet purchased	(11)	0
Certificates of deposit \| Level 1
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned at fair value	481	465
Securities sold, but not yet purchased	(11)
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	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	0
Debt securities
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned at fair value	1,962	1,526
Securities sold, but not yet purchased	(108)	(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	0	0
Debt securities \| Level 2
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned at fair value	1,962	1,526
Securities sold, but not yet purchased	(108)	(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	0	0
U.S. Treasury notes
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned at fair value	103	102
Securities sold, but not yet purchased	(201)	(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	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	103	102
Securities sold, but not yet purchased	(201)	(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	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	1,532	2,856
Securities sold, but not yet purchased	(1)	(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	661	1,981
Securities sold, but not yet purchased	(1)	(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	871	875
Securities sold, but not yet purchased	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	0	0
Other investments
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned at fair value	908	961
Securities sold, but not yet purchased	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	908	961
Other investments \| Level 3
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned at fair value	$ 0	$ 0