Investments of Operating Entities and Consolidated Funds - Securities Sold, Not Yet Purchased (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
US Government Securities [Member]
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	$ 8	[1]	$ 165,197	[1]
Common Stock [Member]
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	195,911		123,877
Operating Entities [Member]
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	210,705		334,251
Operating Entities [Member] | US Government Securities [Member] | Minimum [Member]
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate	5.95%		0.13%
Operating Entities [Member] | US Government Securities [Member] | Maximum [Member]
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate	7.41%		7.41%
Operating Entities [Member] | Corporate Bonds [Member]
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	61	[2]	1,529	[2]
Operating Entities [Member] | Corporate Bonds [Member] | Minimum [Member]
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate	5.55%		5.55%
Operating Entities [Member] | Corporate Bonds [Member] | Maximum [Member]
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate			9.50%
Operating Entities [Member] | Options Held [Member]
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	$ 14,725		$ 43,648

[1] As of September 30, 2012, maturities ranged from April 2016 to January 2040 and interest rates ranged between 5.95% and 7.41%. As of December 31, 2011, maturities ranged from September 2013 to January 2040 and interest rates ranged between 0.13% and 7.41%.

[2] As of September 30, 2012, the maturity was January 2026 with an interest rate of 5.55%. As of December 31, 2011, maturities ranged from December 2016 to January 2026 and interest rates ranged between 5.55% and 9.50%.