Investments of Operating Entities and Consolidated Funds - Securities Owned at Fair Value (Details) (USD $)					3 Months Ended		9 Months Ended						9 Months Ended	12 Months Ended						9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended					9 Months Ended																		12 Months Ended												9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Operating Entities [Member]	Dec. 31, 2011 Operating Entities [Member]	Sep. 30, 2012 Operating Entities [Member] Other Income [Member]	Sep. 30, 2011 Operating Entities [Member] Other Income [Member]	Sep. 30, 2012 Operating Entities [Member] Other Income [Member]	Sep. 30, 2011 Operating Entities [Member] Other Income [Member]	Sep. 30, 2012 Operating Entities [Member] US Government Securities [Member]		Dec. 31, 2011 Operating Entities [Member] US Government Securities [Member]		Sep. 30, 2012 Operating Entities [Member] US Government Securities [Member] Minimum [Member]	Dec. 31, 2011 Operating Entities [Member] US Government Securities [Member] Minimum [Member]	Dec. 31, 2011 Operating Entities [Member] US Government Securities [Member] Maximum [Member]	Sep. 30, 2012 Operating Entities [Member] Corporate Bonds [Member]		Dec. 31, 2011 Operating Entities [Member] Corporate Bonds [Member]		Sep. 30, 2012 Operating Entities [Member] Corporate Bonds [Member] Minimum [Member]	Dec. 31, 2011 Operating Entities [Member] Corporate Bonds [Member] Minimum [Member]	Sep. 30, 2012 Operating Entities [Member] Corporate Bonds [Member] Maximum [Member]	Dec. 31, 2011 Operating Entities [Member] Corporate Bonds [Member] Maximum [Member]	Dec. 31, 2011 Operating Entities [Member] Convertible Bonds [Member]		Sep. 30, 2012 Operating Entities [Member] Convertible Bonds [Member]		Sep. 30, 2012 Operating Entities [Member] Convertible Bonds [Member] Minimum [Member]	Sep. 30, 2012 Operating Entities [Member] Convertible Bonds [Member] Maximum [Member]	Sep. 30, 2012 Operating Entities [Member] Common Stock [Member]	Dec. 31, 2011 Operating Entities [Member] Common Stock [Member]	Sep. 30, 2012 Operating Entities [Member] Options Held [Member]	Dec. 31, 2011 Operating Entities [Member] Options Held [Member]	Sep. 30, 2012 Operating Entities [Member] Derivative [Member] Receivables from Brokers-Dealers and Clearing Organizations [Member]	Dec. 31, 2011 Operating Entities [Member] Derivative [Member] Receivables from Brokers-Dealers and Clearing Organizations [Member]	Sep. 30, 2012 Operating Entities [Member] Warrants and Rights [Member]	Dec. 31, 2011 Operating Entities [Member] Warrants and Rights [Member]	Sep. 30, 2012 Operating Entities [Member] Credit Default Swaps, Futures and Equity Swap Contracts [Member] Other Assets [Member]	Dec. 31, 2011 Operating Entities [Member] Credit Default Swaps, Futures and Equity Swap Contracts [Member] Other Assets [Member]	Sep. 30, 2012 Operating Entities [Member] Futures and Equity Swap Contracts [Member] Accounts Payable, Accrued Expenses and Other Liabilities [Member]	Dec. 31, 2011 Operating Entities [Member] Futures and Equity Swap Contracts [Member] Accounts Payable, Accrued Expenses and Other Liabilities [Member]	Sep. 30, 2012 Operating Entities [Member] Mutual Funds [Member]	Dec. 31, 2011 Operating Entities [Member] Mutual Funds [Member]	Sep. 30, 2012 Consolidated Funds [Member]	Dec. 31, 2011 Consolidated Funds [Member]	Dec. 31, 2011 Consolidated Funds [Member] Corporate Bonds [Member]		Sep. 30, 2012 Consolidated Funds [Member] Corporate Bonds [Member]		Sep. 30, 2012 Consolidated Funds [Member] Commercial Paper [Member]		Dec. 31, 2011 Consolidated Funds [Member] Commercial Paper [Member]		Sep. 30, 2012 Consolidated Funds [Member] Government Sponsored Securities [Member]		Dec. 31, 2011 Consolidated Funds [Member] Government Sponsored Securities [Member]		Sep. 30, 2012 Consolidated Funds [Member] Government Sponsored Securities [Member] Minimum [Member]	Dec. 31, 2011 Consolidated Funds [Member] Government Sponsored Securities [Member] Minimum [Member]	Sep. 30, 2012 Consolidated Funds [Member] Government Sponsored Securities [Member] Maximum [Member]	Dec. 31, 2011 Consolidated Funds [Member] Government Sponsored Securities [Member] Maximum [Member]	Sep. 30, 2012 Enterprise Master [Member]	Dec. 31, 2011 Enterprise Master [Member]	Sep. 30, 2012 Enterprise Master [Member] Debt [Member]	Dec. 31, 2011 Enterprise Master [Member] Debt [Member]	Sep. 30, 2012 Enterprise Master [Member] Common Stock [Member]	Dec. 31, 2011 Enterprise Master [Member] Common Stock [Member]	Sep. 30, 2012 Enterprise Master [Member] Preferred Stock [Member]	Dec. 31, 2011 Enterprise Master [Member] Preferred Stock [Member]	Sep. 30, 2012 Enterprise Master [Member] Private Equity [Member]	Dec. 31, 2011 Enterprise Master [Member] Private Equity [Member]	Sep. 30, 2012 Enterprise Master [Member] Restricted Stock [Member]	Dec. 31, 2011 Enterprise Master [Member] Restricted Stock [Member]	Sep. 30, 2012 Enterprise Master [Member] Rights [Member]	Dec. 31, 2011 Enterprise Master [Member] Rights [Member]	Sep. 30, 2012 Enterprise Master [Member] Warrant [Member]	Dec. 31, 2011 Enterprise Master [Member] Warrant [Member]	Sep. 30, 2012 Enterprise Master [Member] Currency Forwards [Member]	Dec. 31, 2011 Enterprise Master [Member] Currency Forwards [Member]	Sep. 30, 2012 Enterprise Master [Member] Trade Claims [Member]	Dec. 31, 2011 Enterprise Master [Member] Trade Claims [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Derivative, Fair Value, Net																																																														$ 8,000	$ 53,000															$ 8,000	$ 53,000
Debt securities									95,038,000	[1]	182,868,000	[1]				168,021,000	[2]	231,864,000	[2]					18,130,000	[3]	50,854,000	[3]																							1,594,000	[4]	3,927,000	[4]	1,501,000	[5]	2,006,000	[5]
Equity Securities																														300,990,000	250,380,000	26,543,000	55,699,000			4,436,000	2,759,000					2,890,000	3,214,000																					66,000	0	2,353,000	2,173,000	823,000	1,027,000	294,000	276,000	18,000	47,000	1,755,000	2,173,000	3,000	3,000			128,000	128,000
Financial Instruments, Owned, Corporate Debt, at Fair Value																																														401,000	[6]	0	[6]
Securities owned, at fair value	648,772,000	744,914,000	648,772,000	744,914,000																																								3,095,000	6,334,000																	5,440,000	5,827,000
Debt securities, interest rate													0.25%	0.25%	8.00%					4.52%	3.13%	13.50%	13.50%	2.75%				1.38%	5.75%																	0.58%												0.32%	0.32%	1.74%	1.74%
Derivative Assets																																						1,100,000	800,000
Derivative liabilities																																								100,000	800,000
Realized and unrealized gains/(losses) related to derivatives trading activities					1,300,000	800,000	1,100,000	600,000
Collateral posted																																		$ 11,000,000	$ 8,100,000

[1] As of September 30, 2012, maturities ranged from November 2013 to August 2014 with interest rate of 0.25%. As of December 31, 2011, maturities ranged from November 2013 to November 2021 and interest rates ranged between 0.25% and 8%.

[2] As of September 30, 2012, maturities ranged from January 2013 to February 2041 and interest rates ranged between 4.52% and 13.50%. As of December 31, 2011, maturities ranged from January 2012 to February 2041 and interest rates ranged between 3.13% and 13.50%.

[3] As of September 30, 2012, maturities ranged from October 2014 to June 2017 and interest rates ranged between 1.38% and 5.75%. As of December 31, 2011, the maturity was August 2027 with an interest rate of 2.75%.

[4] As of September 30, 2012, commercial paper was purchased at a discount and matures on October 1, 2012. As of December 31, 2011, commercial paper was purchased at a discount and matured on January 3, 2012.

[5] As of September 30, 2012, maturities ranged from October 2012 to February 2014 and interest rates ranged between 0.32% and 1.74%. As of December 31, 2011, maturities ranged from October 2012 to October 2013 and interest rates ranged between 0.32% and 1.74%.

[6] As of December 31, 2011, the maturity was April 2012 with an interest rate of 0.58%.