Investments of Operating Entities and Consolidated Funds - Securities Sold, Not Yet Purchased (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Common Stock
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	$ 215,863		$ 168,797
Operating Entities
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	228,833		177,937
Operating Entities | Corporate Bonds
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	60	[1]	61	[1]
Securities sold, not yet purchased, interest rate	5.55%		5.55%
Operating Entities | Options Held
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	12,907		9,076
Operating Entities | Warrants and Rights
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	$ 3		$ 3

[1] As of March 31, 2013, the maturity was January 2026 with an interest rate of 5.55%. As of December 31, 2012, the maturity was January 2026 with an interest rate of 5.55% .