Investments of Operating Entities and Consolidated Funds - Derivatives (Details)		3 Months Ended
		Mar. 31, 2016 USD ($) contract shares	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($) contract	Mar. 10, 2014 USD ($) $ / shares
Derivative [Line Items]
Payable for derivatives		$ 2,000		$ 0
Receivable on derivatives		125,000		0
Warrants issued			$ 15,200,000
Cost of hedge transaction and warrant, net					$ 20,500,000
Receivable on derivative contracts, at fair value		45,725,000		39,618,000
Payable for derivative contracts, at fair value		$ 25,134,000		21,183,000
Trading days for expiration		80 days
Equity Swaps
Derivative [Line Items]
Receivable on derivatives		$ 61,000
Options
Derivative [Line Items]
Receivable on derivatives		62,000
Futures
Derivative [Line Items]
Receivable on derivative contracts, at fair value		1,289,000		189,000
Payable for derivative contracts, at fair value		1,834,000		101,000
Derivative Asset, Notional Amount		17,143,000		9,416,000
Derivative Liability, Notional Amount		15,401,000		11,995,000
Currency Forwards
Derivative [Line Items]
Payable for derivatives		2,000
Receivable on derivatives		2,000
Receivable on derivative contracts, at fair value		154,000		659,000
Payable for derivative contracts, at fair value		426,000		463,000
Derivative Asset, Notional Amount		56,569,000		67,862,000
Derivative Liability, Notional Amount		43,658,000		44,156,000
Equity Swaps
Derivative [Line Items]
Receivable on derivative contracts, at fair value		811,000		2,327,000
Payable for derivative contracts, at fair value		156,000		71,000
Derivative Asset, Notional Amount		21,012,000		118,488,000
Derivative Liability, Notional Amount		$ 7,236,000		$ 7,605,000
Options
Derivative [Line Items]
Derivative Asset, Number of Instruments Held | contract	[1]	334,903		289,433
Receivable on derivative contracts, at fair value	[1]	$ 38,978,000		$ 31,456,000
Put Option
Derivative [Line Items]
Payable for derivative contracts, at fair value	[2]	$ 22,718,000		$ 20,548,000
Derivative Liability, Number of Instruments Held | contract	[2]	15,775		16,632
Foreign Exchange Option
Derivative [Line Items]
Receivable on derivative contracts, at fair value		$ 4,493,000		$ 4,987,000
Derivative Asset, Notional Amount		280,507,000		283,797,000
Receivables from Brokers-Dealers and Clearing Organizations
Derivative [Line Items]
Collateral posted		13,200,000		27,100,000
Enterprise Master
Derivative [Line Items]
Receivable (payable) on derivative contracts, at fair value		0		(4,000)
Merger Master
Derivative [Line Items]
Receivable on derivative contracts, at fair value		17,895,000		2,511,000
Payable for derivative contracts, at fair value		1,597,000		593,000
Merger Master | Equity Swaps
Derivative [Line Items]
Receivable on derivative contracts, at fair value		1,289,000		1,001,000
Merger Master | Currency forward
Derivative [Line Items]
Receivable on derivative contracts, at fair value		24,000		235,000
Payable for derivative contracts, at fair value		21,000		0
Merger Master | Options
Derivative [Line Items]
Receivable on derivative contracts, at fair value		16,582,000		1,275,000
Merger Master | Equity Swaps
Derivative [Line Items]
Payable for derivative contracts, at fair value		283,000		30,000
Merger Master | Put Option
Derivative [Line Items]
Payable for derivative contracts, at fair value		$ 1,293,000		563,000
Common Stock Class A
Derivative [Line Items]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights | shares		28,048,786
Class of Warrant, Exercise Price of Warrants or Rights | $ / shares					$ 7.18
Convertible Debt
Derivative [Line Items]
Call Option, Fair Value		$ 20,900,000			$ 35,700,000
Receivable on derivatives contracts, at fair value [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset		45,725,000		39,618,000
Derivative Asset, Fair Value, Amount Offset Against Collateral	[3]	0		0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	[4]	7,305,000		9,339,000
Derivative asset, net of offset		38,420,000		30,279,000
Receivable on derivative contracts, at fair value		45,725,000		39,618,000
Financial Instruments, Owned and Pledged as Collateral, at Fair Value		0		0
Payable for derivatives contracts, at fair value [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability		25,134,000		21,183,000
Payable for derivative contracts, at fair value		25,134,000		21,183,000
Financial Instruments, Owned and Pledged as Collateral, at Fair Value		0		0
Derivative Liability, Fair Value, Amount Offset Against Collateral	[3]	0		0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	[4]	582,000		534,000
Derivative Liability, net of offset		24,552,000		20,649,000
Currency Forwards | Enterprise Master
Derivative [Line Items]
Receivable (payable) on derivative contracts, at fair value		0		$ (4,000)
Other Income [Member]
Derivative [Line Items]
Realized and unrealized gains/(losses) related to derivatives trading activities		$ 2,400,000	$ 2,100,000

[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.