Investments of Operating Entities and Consolidated Funds - Securities Owned at Fair Value (Details) (USD $)					3 Months Ended						3 Months Ended								3 Months Ended									3 Months Ended																								3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Operating Entities	Dec. 31, 2012 Operating Entities	Mar. 31, 2013 Operating Entities Other Income	Mar. 31, 2012 Operating Entities Other Income	Mar. 31, 2013 Operating Entities US Government Securities		Dec. 31, 2012 Operating Entities US Government Securities		Mar. 31, 2013 Operating Entities US Government Securities Minimum	Mar. 31, 2012 Operating Entities US Government Securities Minimum	Mar. 31, 2013 Operating Entities US Government Securities Maximum	Mar. 31, 2012 Operating Entities US Government Securities Maximum	Mar. 31, 2013 Operating Entities Corporate Bonds		Dec. 31, 2012 Operating Entities Corporate Bonds		Mar. 31, 2013 Operating Entities Corporate Bonds Minimum	Mar. 31, 2012 Operating Entities Corporate Bonds Minimum	Mar. 31, 2013 Operating Entities Corporate Bonds Maximum	Mar. 31, 2012 Operating Entities Corporate Bonds Maximum	Mar. 31, 2012 Operating Entities Convertible Bonds	Mar. 31, 2013 Operating Entities Convertible Bonds		Dec. 31, 2012 Operating Entities Convertible Bonds		Mar. 31, 2013 Operating Entities Convertible Bonds Minimum	Mar. 31, 2013 Operating Entities Convertible Bonds Maximum	Mar. 31, 2013 Operating Entities Common Stock	Dec. 31, 2012 Operating Entities Common Stock	Mar. 31, 2013 Operating Entities Preferred Stock	Dec. 31, 2012 Operating Entities Preferred Stock	Mar. 31, 2013 Operating Entities Options Held	Dec. 31, 2012 Operating Entities Options Held	Mar. 31, 2013 Operating Entities Warrants and Rights	Dec. 31, 2012 Operating Entities Warrants and Rights	Mar. 31, 2013 Operating Entities Derivative contracts, at fair value Receivables from Brokers-Dealers and Clearing Organizations	Dec. 31, 2012 Operating Entities Derivative contracts, at fair value Receivables from Brokers-Dealers and Clearing Organizations	Mar. 31, 2013 Operating Entities Credit Default Swaps, Futures and Equity Swap Contracts Other	Dec. 31, 2012 Operating Entities Credit Default Swaps, Futures and Equity Swap Contracts Other	Mar. 31, 2013 Operating Entities Futures and Equity Swap Contracts Accounts Payable, Accrued Expenses and Other Liabilities	Dec. 31, 2012 Operating Entities Futures and Equity Swap Contracts Accounts Payable, Accrued Expenses and Other Liabilities	Mar. 31, 2013 Operating Entities Mutual Funds	Dec. 31, 2012 Operating Entities Mutual Funds	Mar. 31, 2013 Consolidated Funds	Dec. 31, 2012 Consolidated Funds	Dec. 31, 2012 Consolidated Funds Commercial Paper		Dec. 31, 2012 Consolidated Funds Government Sponsored Securities		Mar. 31, 2012 Consolidated Funds Government Sponsored Securities Minimum	Mar. 31, 2012 Consolidated Funds Government Sponsored Securities Maximum	Mar. 31, 2013 Enterprise Master	Dec. 31, 2012 Enterprise Master	Mar. 31, 2013 Enterprise Master Debt	Dec. 31, 2012 Enterprise Master Debt	Mar. 31, 2013 Enterprise Master Common Stock	Dec. 31, 2012 Enterprise Master Common Stock	Mar. 31, 2013 Enterprise Master Preferred Stock	Dec. 31, 2012 Enterprise Master Preferred Stock	Mar. 31, 2013 Enterprise Master Private Equity	Dec. 31, 2012 Enterprise Master Private Equity	Mar. 31, 2013 Enterprise Master Restricted Stock	Dec. 31, 2012 Enterprise Master Restricted Stock	Mar. 31, 2013 Enterprise Master Rights	Dec. 31, 2012 Enterprise Master Rights	Mar. 31, 2013 Enterprise Master Warrants and Rights	Dec. 31, 2012 Enterprise Master Warrants and Rights	Mar. 31, 2013 Enterprise Master Currency Forwards	Dec. 31, 2012 Enterprise Master Currency Forwards	Mar. 31, 2013 Enterprise Master Trade Claims	Dec. 31, 2012 Enterprise Master Trade Claims
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other investments	$ 80,452,000	$ 84,930,000	$ 80,452,000	$ 84,930,000																																										$ 193,497,000	$ 204,205,000
Derivative, Fair Value, Net																																																						5,000	6,000															5,000	6,000
Debt securities							134,000	[1]	137,478,000	[1]					143,446,000	[2]	193,078,000	[2]						8,615,000	[3]	6,202,000	[3]					2,332,000	2,332,000															1,614,000	[4]	1,911,000	[5]
Equity Securities																														293,067,000	259,292,000			18,908,000	20,546,000	3,343,000	2,354,000							2,953,000	2,845,000											106,000	79,000	2,682,000	2,680,000	997,000	997,000	297,000	297,000	110,000	26,000	1,665,000	1,714,000	2,000	2,000			128,000	128,000
Securities owned, at fair value	472,798,000	624,127,000	472,798,000	624,127,000																																											3,525,000							5,987,000	5,923,000
Debt securities, interest rate											0.11%	0.25%	5.95%	5.95%					2.47%	3.09%	12.50%	12.50%	5.00%					5.00%	6.25%																							0.28%	4.00%
Derivative Assets																																								300,000	200,000
Derivative liabilities																																										200,000	1,000,000
Realized and unrealized gains/(losses) related to derivatives trading activities					1,600,000	600,000
Collateral posted																																						$ 3,400,000	$ 6,700,000

[1] As of March 31, 2013, maturities ranged from September 2013 to April 2016 and interest rates ranged between 0.11% and 5.95%. As of December 31, 2012, maturities ranged from November 2013 to November 2022 and interest rates ranged between 0.25% and 5.95%.

[2] As of March 31, 2013, maturities ranged from April 2013 to February 2041 and interest rates ranged between 2.47% and 12.50%. As of December 31, 2012, maturities ranged from January 2013 to February 2041 and interest rates ranged between 3.09% and 12.50%.

[3] As of March 31, 2013, maturities ranged from May 2014 to October 2014 with an interest rate of 5.00% to 6.25%. As of December 31, 2012, maturities ranged from May 2014 to July 2014 with an interest rate of 5.00%.

[4] As of December 31, 2012, commercial paper was purchased at a discount and matures on January 2, 2013.

[5] As of December 31, 2012, maturities ranged from August 2013 to December 2014 and interest rates ranged between 0.28% and 4.00%.