Investments of Operating Entities and Consolidated Funds - Derivatives (Details) (USD $)	6 Months Ended																									3 Months Ended		6 Months Ended
	Jun. 30, 2014 contract		Mar. 10, 2014	Dec. 31, 2013 contract		Jun. 30, 2014 Futures	Dec. 31, 2013 Futures	Jun. 30, 2014 Forward Contracts	Dec. 31, 2013 Forward Contracts	Jun. 30, 2014 Cross Rate	Dec. 31, 2013 Cross Rate	Jun. 30, 2014 Equity Swaps		Dec. 31, 2013 Equity Swaps		Jun. 30, 2014 Options		Dec. 31, 2013 Options		Jun. 30, 2014 Put Option		Dec. 31, 2013 Put Option		Jun. 30, 2014 Receivables from Brokers-Dealers and Clearing Organizations	Dec. 31, 2013 Receivables from Brokers-Dealers and Clearing Organizations	Jun. 30, 2014 Other Income	Jun. 30, 2013 Other Income	Jun. 30, 2014 Other Income	Jun. 30, 2013 Other Income	Jun. 30, 2014 Merger Master	Dec. 31, 2013 Merger Master	Jun. 30, 2014 Common Stock	Mar. 10, 2014 Common Stock
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 45,043,000			$ 10,075,000
Derivative, Fair Value, Net																														(1,080,000)	(156,000)
Embedded cash conversion value	37,700,000		35,700,000
Call Option, Fair Value	37,700,000		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	45,043,000			10,075,000		92,000	285,000			0	22,000	55,000	[1]	70,000	[1]	44,896,000	[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	45,043,000	[4]		10,075,000	[4]
Derivative asset, net of offset	0			0
Derivative Liability, Fair Value, Gross Liability	53,353,000			7,674,000
Derivative Asset, Number of Instruments Held	58,555			71,129
Derivative liabilities	53,353,000			7,674,000		11,000	275,000	156,000	301,000			2,280,000	[1]	525,000	[1]					50,906,000	[5]	6,573,000	[5]
Derivative Liability, Number of Instruments Held	28,483			38,221
Derivative Asset, Notional Amount	1,000,000			200,000
Derivative Liability, Notional Amount	23,000,000			100,000
Realized and unrealized gains/(losses) related to derivatives trading activities																										(1,800,000)	800,000	(3,700,000)	(4,400,000)
Collateral posted																								19,200,000	10,000,000
Derivative Liability, Fair Value, Amount Offset Against Collateral	0	[3]		0	[3]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	53,353,000	[4]		7,674,000	[4]
Derivative Liability, net of offset	$ 0			$ 0
Trading days for expiration	80 days

[1] The notional values of equity swaps classified as receivable on derivative contracts are $0.6 million and $0.2 million as of June 30, 2014 and December 31, 2013, respectively. The notional values of equity and credit default swaps classified as payable for derivative contracts are $23.0 million and $0.1 million as of June 30, 2014 and December 31, 2013, respectively.

[2] As of June 30, 2014 and December 31, 2013, the Company had 58,555 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] As of June 30, 2014 and December 31, 2013, the Company had 28,483 and 38,221 contracts held, respectively.