Investments of Operating Entities and Consolidated Funds - Derivatives (Details) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2016 USD ($) contract shares	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) contract shares	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) contract	Mar. 10, 2014 USD ($) $ / shares
Derivative [Line Items]
Payable for derivatives		$ 276		$ 276		$ 0
Receivable on derivatives		429		$ 429		0
Warrants issued					$ 15,200
Cost of hedge transaction and warrant, net							$ 20,500
Trading days for expiration				80 days
Receivable on derivative contracts, at fair value		27,428		$ 27,428		39,618
Payable for derivative contracts, at fair value		17,021		17,021		21,183
Equity Swaps
Derivative [Line Items]
Receivable on derivatives		366		366
Receivable on derivative contracts, at fair value		1,442		1,442		2,327
Payable for derivative contracts, at fair value		0		0		71
Derivative Asset, Notional Amount		67,059		67,059		118,488
Derivative Liability, Notional Amount		0		0		7,605
Futures
Derivative [Line Items]
Payable for derivatives		246		246
Receivable on derivative contracts, at fair value		704		704		189
Payable for derivative contracts, at fair value		1,235		1,235		101
Derivative Asset, Notional Amount		13,935		13,935		9,416
Derivative Liability, Notional Amount		42,736		42,736		11,995
Currency Forwards
Derivative [Line Items]
Receivable on derivatives		1		1
Receivable on derivative contracts, at fair value		17		17		659
Payable for derivative contracts, at fair value		37		37		463
Derivative Asset, Notional Amount		59,902		59,902		67,862
Derivative Liability, Notional Amount		24,650		24,650		44,156
Options
Derivative [Line Items]
Receivable on derivatives		62		62
Receivable on derivative contracts, at fair value	[1]	$ 22,941		$ 22,941		$ 31,456
Derivative Asset, Number of Instruments Held | contract	[1]	242,968		242,968		289,433
Put Option
Derivative [Line Items]
Payable for derivatives		$ 30		$ 30
Payable for derivative contracts, at fair value	[2]	$ 15,749		$ 15,749		$ 20,548
Derivative Liability, Number of Instruments Held | contract	[2]	11,235		11,235		16,632
Foreign Exchange Option
Derivative [Line Items]
Receivable on derivative contracts, at fair value		$ 2,324		$ 2,324		$ 4,987
Derivative Asset, Notional Amount		245,700		245,700		283,797
Receivables from Brokers-Dealers and Clearing Organizations
Derivative [Line Items]
Collateral posted		23,100		23,100		27,100
Enterprise Master
Derivative [Line Items]
Receivable (payable) on derivative contracts, at fair value		0		0		(4)
Merger Master
Derivative [Line Items]
Receivable on derivative contracts, at fair value		5,512		5,512		2,511
Payable for derivative contracts, at fair value		1,586		1,586		593
Merger Master | Equity Swaps
Derivative [Line Items]
Receivable on derivative contracts, at fair value		2,525		2,525		1,001
Payable for derivative contracts, at fair value		0		0		30
Merger Master | Currency Forwards
Derivative [Line Items]
Receivable on derivative contracts, at fair value		0		0		235
Merger Master | Options
Derivative [Line Items]
Receivable on derivative contracts, at fair value		2,987		2,987		1,275
Merger Master | Put Option
Derivative [Line Items]
Payable for derivative contracts, at fair value		$ 1,586		$ 1,586		563
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		$ 13,000		$ 13,000			$ 35,700
Receivable on derivatives contracts, at fair value [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset		27,428		27,428		39,618
Derivative Asset, Fair Value, Amount Offset Against Collateral	[3]	0		0		0
Receivable on derivative contracts, at fair value		27,428		27,428		39,618
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	[4]	4,278		4,278		9,339
Derivative asset, net of offset		23,150		23,150		30,279
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		17,021		17,021		21,183
Derivative Liability, Fair Value, Amount Offset Against Collateral	[3]	0		0		0
Payable for derivative contracts, at fair value		17,021		17,021		21,183
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	[4]	37		37		534
Derivative Liability, net of offset		16,984		16,984		20,649
Financial Instruments, Owned and Pledged as Collateral, at Fair Value		0		0		0
Other Income [Member]
Derivative [Line Items]
Realized and unrealized gains/(losses) related to derivatives trading activities		(2,800)	$ (200)	(10,400)	$ (5,600)
Currency Forwards | Enterprise Master
Derivative [Line Items]
Receivable (payable) on derivative contracts, at fair value		$ 0		$ 0		$ (4)

[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.