Investments of Operating Entities and Consolidated Funds - Derivatives (Details) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2016 USD ($) contract shares	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) contract shares	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) contract	Mar. 10, 2014 USD ($) $ / shares
Derivative [Line Items]
Receivable on derivatives		$ 60		$ 60		$ 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		24,971		$ 24,971		39,618
Payable for derivative contracts, at fair value		14,856		14,856		21,183
Equity Swaps
Derivative [Line Items]
Receivable on derivatives		23		23
Options
Derivative [Line Items]
Receivable on derivatives		19		19
Futures
Derivative [Line Items]
Receivable on derivative contracts, at fair value		797		797		189
Payable for derivative contracts, at fair value		1,005		1,005		101
Derivative Asset, Notional Amount		19,282		19,282		9,416
Derivative Liability, Notional Amount		33,424		33,424		11,995
Currency Forwards
Derivative [Line Items]
Receivable on derivatives		18		18
Receivable on derivative contracts, at fair value		160		160		659
Payable for derivative contracts, at fair value		403		403		463
Derivative Asset, Notional Amount		19,354		19,354		67,862
Derivative Liability, Notional Amount		86,059		86,059		44,156
Equity Swaps
Derivative [Line Items]
Receivable on derivative contracts, at fair value		2,654		2,654		2,327
Payable for derivative contracts, at fair value		2,408		2,408		71
Derivative Asset, Notional Amount		100,870		100,870		118,488
Derivative Liability, Notional Amount		65,381		65,381		7,605
Options
Derivative [Line Items]
Receivable on derivative contracts, at fair value	[1]	$ 17,841		$ 17,841		$ 31,456
Derivative Asset, Number of Instruments Held | contract	[1]	262,443		262,443		289,433
Put Option
Derivative [Line Items]
Payable for derivative contracts, at fair value	[2]	$ 11,040		$ 11,040		$ 20,548
Derivative Liability, Number of Instruments Held | contract	[2]	9,606		9,606		16,632
Foreign Exchange Option
Derivative [Line Items]
Receivable on derivative contracts, at fair value		$ 3,519		$ 3,519		$ 4,987
Derivative Asset, Notional Amount		252,038		252,038		283,797
Receivables from Brokers-Dealers and Clearing Organizations
Derivative [Line Items]
Collateral posted		36,200		36,200		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		6,198		6,198		2,511
Payable for derivative contracts, at fair value		4,192		4,192		593
Merger Master | Equity Swaps
Derivative [Line Items]
Receivable on derivative contracts, at fair value		4,122		4,122		1,001
Merger Master | Options
Derivative [Line Items]
Receivable on derivative contracts, at fair value		2,036		2,036		1,275
Merger Master | Currency Forwards
Derivative [Line Items]
Receivable on derivative contracts, at fair value		40		40		235
Payable for derivative contracts, at fair value		355		355		0
Merger Master | Equity Swaps
Derivative [Line Items]
Payable for derivative contracts, at fair value		3,837		3,837		30
Merger Master | Put Option
Derivative [Line Items]
Payable for derivative contracts, at fair value		$ 0		$ 0		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		$ 8,500		$ 8,500			$ 35,700
Receivable on derivatives contracts, at fair value [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset		24,971		24,971		39,618
Derivative Asset, Fair Value, Amount Offset Against Collateral	[3]	0		0		0
Receivable on derivative contracts, at fair value		24,971		24,971		39,618
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	[4]	7,674		7,674		9,339
Derivative asset, net of offset		17,297		17,297		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		14,856		14,856		21,183
Derivative Liability, Fair Value, Amount Offset Against Collateral	[3]	0		0		0
Payable for derivative contracts, at fair value		14,856		14,856		21,183
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	[4]	2,811		2,811		534
Derivative Liability, net of offset		12,045		12,045		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		(9,900)	$ (7,500)	(7,500)	$ (5,400)
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.