Investments of Operating Entities and Consolidated Funds - Derivatives (Details) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2015 USD ($) contract shares	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) contract shares	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($) contract	Mar. 10, 2014 USD ($) $ / shares
Derivative [Line Items]
Securities sold, not yet purchased, at fair value		$ 250,739		$ 250,739		$ 207,875
Warrants issued					$ 15,218
Cost of hedge transaction and warrant, net							$ 20,500
Receivable on derivative contracts, at fair value		41,587		41,587		49,877
Payable for derivative contracts, at fair value		26,078		$ 26,078		41,330
Trading days for expiration				80 days
Futures
Derivative [Line Items]
Receivable on derivative contracts, at fair value		68		$ 68		75
Payable for derivative contracts, at fair value		381		381		33
Derivative Asset, Notional Amount		4,679		4,679		3,041
Derivative Liability, Notional Amount		32,423		32,423		2,213
Currency Forwards
Derivative [Line Items]
Receivable on derivative contracts, at fair value		105		105		310
Payable for derivative contracts, at fair value		107		107		0
Derivative Asset, Notional Amount		34,036		34,036		23,961
Derivative Liability, Notional Amount		43,788		43,788		0
Equity Swaps
Derivative [Line Items]
Receivable on derivative contracts, at fair value		470		470		251
Payable for derivative contracts, at fair value		2,615		2,615		1,603
Derivative Asset, Notional Amount		52,000		52,000		12,904
Derivative Liability, Notional Amount		45,193		45,193		18,352
Options
Derivative [Line Items]
Receivable on derivative contracts, at fair value	[1]	$ 34,498		$ 34,498		$ 48,201
Derivative Asset, Number of Instruments Held | contract	[1]	306,659		306,659		367,441
Put Option
Derivative [Line Items]
Payable for derivative contracts, at fair value	[2]	$ 22,975		$ 22,975		$ 39,694
Derivative Liability, Number of Instruments Held | contract	[2]	14,035		14,035		22,043
Foreign Exchange Option
Derivative [Line Items]
Receivable on derivative contracts, at fair value		$ 6,446		$ 6,446		$ 1,040
Derivative Asset, Notional Amount		292,475		292,475		32,200
Receivables from Brokers-Dealers and Clearing Organizations
Derivative [Line Items]
Collateral posted		20,900		20,900		5,500
Other Income
Derivative [Line Items]
Realized and unrealized gains/(losses) related to derivatives trading activities		(200)	$ 5,100	(5,600)	$ 1,400
Enterprise Master
Derivative [Line Items]
Receivable on derivative contracts, at fair value		(4)		(4)		64
Merger Master
Derivative [Line Items]
Receivable on derivative contracts, at fair value		776		776		617
Payable for derivative contracts, at fair value		4,699		4,699		294
Merger Master | Exchange Traded Funds
Derivative [Line Items]
Payable for derivative contracts, at fair value		3,801		3,801		0
Merger Master | Equity Swaps
Derivative [Line Items]
Payable for derivative contracts, at fair value		862		862		56
Merger Master | Put Option
Derivative [Line Items]
Payable for derivative contracts, at fair value		36		36		238
Quadratic Master
Derivative [Line Items]
Receivable on derivative contracts, at fair value		$ 9,654		$ 9,654
Common Stock Class A
Derivative [Line Items]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights | shares		28,048,786		28,048,786
Class of Warrant, Exercise Price of Warrants or Rights | $ / shares							$ 7.18
Convertible Debt
Derivative [Line Items]
Call Option, Fair Value		$ 21,600		$ 21,600			$ 35,700
Receivable on derivatives contracts, at fair value [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset		41,587		41,587		49,877
Receivable on derivative contracts, at fair value		41,587		41,587		49,877
Derivative Asset, Fair Value, Amount Offset Against Collateral	[3]	0		0		0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	[4]	8,950		8,950		2,588
Derivative asset, net of offset		32,637		32,637		47,289
Financial Instruments, Owned and Pledged as Collateral, at Fair Value		0		0		0
Payable for derivatives contracts, at fair value [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability		26,078		26,078		41,330
Payable for derivative contracts, at fair value		26,078		26,078		41,330
Financial Instruments, Owned and Pledged as Collateral, at Fair Value		0		0		0
Derivative Liability, Fair Value, Amount Offset Against Collateral	[3]	0		0		0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	[4]	2,643		2,643		1,603
Derivative Liability, net of offset		23,435		23,435		39,727
Currency forward | Quadratic Master
Derivative [Line Items]
Receivable on derivative contracts, at fair value		32		32
Options | Quadratic Master
Derivative [Line Items]
Receivable on derivative contracts, at fair value		9,622		9,622
Put Option | Quadratic Master
Derivative [Line Items]
Payable for derivative contracts, at fair value		100		100
Currency Forwards | Enterprise Master
Derivative [Line Items]
Receivable on derivative contracts, at fair value		$ (4)		$ (4)		$ 64

[1] Includes index, equity, commodity future and cash conversion options.

[2] Includes index, equity, commodity future and cash conversion options.

[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.