Investments of Operating Entities and Consolidated Funds - Derivatives (Details) $ / shares in Units, $ in Thousands		12 Months Ended
		Dec. 31, 2015 USD ($) contract shares	Dec. 31, 2014 USD ($) contract	Dec. 31, 2013 USD ($)	Mar. 10, 2014 USD ($) $ / shares
Derivative [Line Items]
Securities sold, not yet purchased, at fair value		$ 257,159	$ 207,875
Warrants issued			15,218
Cost of hedge transaction and warrant, net					$ 20,500
Receivable on derivative contracts, at fair value		39,618	49,877
Payable for derivative contracts, at fair value		$ 21,183	41,330
Trading days for expiration		80 days
Futures
Derivative [Line Items]
Receivable on derivative contracts, at fair value		$ 189	75
Payable for derivative contracts, at fair value		101	33
Derivative Asset, Notional Amount		9,416	3,041
Derivative Liability, Notional Amount		11,995	2,213
Currency Forwards
Derivative [Line Items]
Receivable on derivative contracts, at fair value		659	310
Payable for derivative contracts, at fair value		463	0
Derivative Asset, Notional Amount		67,862	23,961
Derivative Liability, Notional Amount		44,156	0
Equity Swaps
Derivative [Line Items]
Receivable on derivative contracts, at fair value		2,327	251
Payable for derivative contracts, at fair value		71	1,603
Derivative Asset, Notional Amount		118,488	12,904
Derivative Liability, Notional Amount		7,605	18,352
Options
Derivative [Line Items]
Receivable on derivative contracts, at fair value	[1]	$ 31,456	$ 48,201
Derivative Asset, Number of Instruments Held | contract	[1]	289,433	367,441
Put Option
Derivative [Line Items]
Payable for derivative contracts, at fair value	[2]	$ 20,548	$ 39,694
Derivative Liability, Number of Instruments Held | contract	[2]	16,632	22,043
Foreign Exchange Option
Derivative [Line Items]
Receivable on derivative contracts, at fair value		$ 4,987	$ 1,040
Derivative Asset, Notional Amount		283,797	32,200
Receivables from Brokers-Dealers and Clearing Organizations
Derivative [Line Items]
Collateral posted		27,100	5,500
Other Income
Derivative [Line Items]
Realized and unrealized gains/(losses) related to derivatives trading activities		(4,600)	(500)	$ 4,400
Enterprise Master
Derivative [Line Items]
Receivable on derivative contracts, at fair value		(4)	64
Merger Master
Derivative [Line Items]
Receivable on derivative contracts, at fair value		2,511	619
Payable for derivative contracts, at fair value		593	296
Merger Master | Equity Swaps
Derivative [Line Items]
Receivable on derivative contracts, at fair value		1,001	78
Merger Master | Currency forward
Derivative [Line Items]
Receivable on derivative contracts, at fair value		235	0
Merger Master | Options
Derivative [Line Items]
Receivable on derivative contracts, at fair value		1,275	541
Merger Master | Equity Swaps
Derivative [Line Items]
Payable for derivative contracts, at fair value		30	58
Merger Master | Put Option
Derivative [Line Items]
Payable for derivative contracts, at fair value		563	238
Quadratic Master
Derivative [Line Items]
Receivable on derivative contracts, at fair value		$ 9,099
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		$ 18,200			$ 35,700
Receivable on derivatives contracts, at fair value [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset		39,618	49,877
Receivable on derivative contracts, at fair value		39,618	49,877
Derivative Asset, Fair Value, Amount Offset Against Collateral	[3]	0	0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	[4]	9,339	2,588
Derivative asset, net of offset		30,279	47,289
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		21,183	41,330
Payable for derivative contracts, at fair value		21,183	41,330
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]	534	1,603
Derivative Liability, net of offset		20,649	39,727
Currency forward | Quadratic Master
Derivative [Line Items]
Receivable on derivative contracts, at fair value		92
Options | Quadratic Master
Derivative [Line Items]
Receivable on derivative contracts, at fair value		9,007
Put Option | Quadratic Master
Derivative [Line Items]
Payable for derivative contracts, at fair value		200
Currency Forwards | Enterprise Master
Derivative [Line Items]
Receivable on derivative contracts, at fair value		$ (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.