Investments of Operating Entities and Consolidated Funds - Derivatives (Details) (USD $)	9 Months Ended																									3 Months Ended		9 Months Ended
	Sep. 30, 2014 contract		Mar. 10, 2014	Dec. 31, 2013 contract		Sep. 30, 2014 Futures	Dec. 31, 2013 Futures	Sep. 30, 2014 Forward Contracts	Dec. 31, 2013 Forward Contracts	Sep. 30, 2014 Cross Rate	Dec. 31, 2013 Cross Rate	Sep. 30, 2014 Equity Swaps		Dec. 31, 2013 Equity Swaps		Sep. 30, 2014 Options		Dec. 31, 2013 Options		Sep. 30, 2014 Put Option		Dec. 31, 2013 Put Option		Sep. 30, 2014 Receivables from Brokers-Dealers and Clearing Organizations	Dec. 31, 2013 Receivables from Brokers-Dealers and Clearing Organizations	Sep. 30, 2014 Other Income	Sep. 30, 2013 Other Income	Sep. 30, 2014 Other Income	Sep. 30, 2013 Other Income	Sep. 30, 2014 Merger Master	Dec. 31, 2013 Merger Master	Sep. 30, 2014 Common Stock Class A	Mar. 10, 2014 Common Stock Class A
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 35,966,000			$ 10,075,000
Derivative, Fair Value, Net																														404,000	(156,000)
Embedded cash conversion value	28,800,000		35,700,000
Call Option, Fair Value	28,800,000		35,700,000
Warrants issued	15,218,000
Cost of hedge transaction and warrant, net			20,500,000
Class of Warrant or Right, Number of Securities Called by Warrants or Rights																																28,048,786
Class of Warrant, Exercise Price of Warrants or Rights																																	7.18
Derivative Assets	35,966,000			10,075,000		177,000	285,000			291,000	22,000	1,541,000	[1]	70,000	[1]	33,957,000	[2]	9,698,000	[2]
Derivative Asset, Fair Value, Amount Offset Against Collateral	0	[3]		0	[3]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	1,832,000	[4]		92,000	[4]
Derivative asset, net of offset	34,134,000			9,983,000
Derivative Liability, Fair Value, Gross Liability	31,369,000			7,674,000
Derivative Asset, Number of Instruments Held	349,989			71,129
Derivative liabilities	31,369,000			7,674,000		24,000	275,000	0	301,000			51,000	[5]	525,000	[5]					31,294,000	[6]	6,573,000	[6]
Financial Instruments, Owned and Pledged as Collateral, at Fair Value	0			0
Derivative Liability, Number of Instruments Held	20,224			38,221
Derivative Asset, Notional Amount	5,600,000			21,400,000
Derivative Liability, Notional Amount	5,500,000			16,500,000
Realized and unrealized gains/(losses) related to derivatives trading activities																										5,100,000	800,000	1,400,000	(3,600,000)
Collateral posted																								14,100,000	10,000,000
Derivative Liability, Fair Value, Amount Offset Against Collateral	0	[3]		0	[3]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	51,000	[4]		826,000	[4]
Derivative Liability, net of offset	$ 31,318,000			$ 6,848,000
Trading days for expiration	80 days

[1] The notional values of equity swaps classified as receivable on derivative contracts are $5.6 million and $21.4 million as of September 30, 2014 and December 31, 2013, respectively.

[2] As of September 30, 2014 and December 31, 2013, the Company had 349,989 and 71,129 contracts held, respectively.

[3] Includes financial instruments subject to enforceable master netting provisions that are permitted to be offset to the extent an event of default has occurred.

[4] Includes the amount of collateral held or posted.

[5] The notional values of equity and credit default swaps classified as payable for derivative contracts are $5.5 million and $16.5 million as of September 30, 2014 and December 31, 2013, respectively.

[6] As of September 30, 2014 and December 31, 2013, the Company had 20,224 and 38,221 contracts held, respectively.