Investments of Operating Entities and Consolidated Funds - Derivatives (Details) $ / shares in Units, $ in Thousands			3 Months Ended			9 Months Ended
		Dec. 05, 2016	Sep. 30, 2017 USD ($) contract shares	Sep. 30, 2016 USD ($)	Mar. 31, 2015 USD ($)	Sep. 30, 2017 USD ($) contract shares	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) contract	Mar. 10, 2014 USD ($) $ / shares
Derivative [Line Items]
Payable for derivative contracts, at fair value, Consolidated Funds			$ 1,747			$ 1,747		$ 572
Receivable on derivatives, Consolidated Funds			1,042			$ 1,042		893
Stockholders' Equity Note, Stock Split, Conversion Ratio		0.25
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			32,105			$ 32,105		22,901
Payable for derivative contracts, at fair value			26,317			26,317		20,762
Futures
Derivative [Line Items]
Receivable on derivative contracts, at fair value			15			15		104
Payable for derivative contracts, at fair value			175			175		642
Derivative Asset, Notional Amount			2,972			2,972		12,421
Derivative Liability, Notional Amount			12,330			12,330		38,345
Swaps
Derivative [Line Items]
Receivable on derivative contracts, at fair value			4,923			4,923		468
Payable for derivative contracts, at fair value			1,693			1,693		181
Derivative Asset, Notional Amount			74,571			74,571		46,462
Derivative Liability, Notional Amount			75,595			75,595		9,533
Currency forward
Derivative [Line Items]
Payable for derivative contracts, at fair value, Consolidated Funds			551			551		10
Receivable on derivatives, Consolidated Funds			3			3		18
Equity Swap
Derivative [Line Items]
Payable for derivative contracts, at fair value, Consolidated Funds			1,132			1,132		495
Receivable on derivatives, Consolidated Funds			809			809		731
Currency Forwards
Derivative [Line Items]
Receivable on derivative contracts, at fair value			186			186		592
Payable for derivative contracts, at fair value			18			18		0
Derivative Asset, Notional Amount			81,288			81,288		80,608
Derivative Liability, Notional Amount			32,443			32,443		0
Options
Derivative [Line Items]
Payable for derivative contracts, at fair value, Consolidated Funds			64			64		67
Receivable on derivatives, Consolidated Funds			230			230		144
Receivable on derivative contracts, at fair value	[1]		$ 26,757			$ 26,757		$ 21,539
Derivative Asset, Number of Instruments Held | contract	[1]		284,793			284,793		256,097
Put Option
Derivative [Line Items]
Payable for derivative contracts, at fair value	[2]		$ 24,431			$ 24,431		$ 19,939
Derivative Liability, Number of Instruments Held | contract	[2]		47,564			47,564		23,726
Foreign Exchange Option
Derivative [Line Items]
Receivable on derivative contracts, at fair value			$ 224			$ 224		$ 198
Derivative Asset, Notional Amount			44,201			44,201		57,051
Receivables from Brokers-Dealers and Clearing Organizations
Derivative [Line Items]
Collateral posted			17,200			17,200		17,100
Receivable on derivatives contracts, at fair value [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset			32,105			32,105		22,901
Derivative Asset, Fair Value, Amount Offset Against Collateral	[3]		0			0		0
Receivable on derivative contracts, at fair value			32,105			32,105		22,901
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	[4]		5,414			5,414		1,382
Derivative asset, net of offset			26,691			26,691		21,519
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			26,317			26,317		20,762
Derivative Liability, Fair Value, Amount Offset Against Collateral	[3]		0			0		0
Payable for derivative contracts, at fair value			26,317			26,317		20,762
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	[4]		1,710			1,710		181
Derivative Liability, net of offset			24,607			24,607		20,581
Financial Instruments, Owned and Pledged as Collateral, at Fair Value			0			0		0
Merger Master
Derivative [Line Items]
Receivable on derivative contracts, at fair value			7,689			7,689		4,519
Payable for derivative contracts, at fair value			4,401			4,401		2,408
Merger Master | Swaps
Derivative [Line Items]
Receivable on derivative contracts, at fair value			3,012			3,012		255
Payable for derivative contracts, at fair value			3,540			3,540		123
Merger Master | Currency Forwards
Derivative [Line Items]
Receivable on derivative contracts, at fair value			82			82		0
Merger Master | Options
Derivative [Line Items]
Receivable on derivative contracts, at fair value			4,595			4,595		4,264
Merger Master | Put Option
Derivative [Line Items]
Payable for derivative contracts, at fair value			$ 861			$ 861		$ 2,285
Common Stock Class A
Derivative [Line Items]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights | shares			7,012,196			7,012,196
Class of Warrant, Exercise Price of Warrants or Rights | $ / shares									$ 28.72
Convertible Debt
Derivative [Line Items]
Call Option, Fair Value			$ 10,400			$ 10,400			$ 35,700
Other Income [Member]
Derivative [Line Items]
Realized and unrealized gains/(losses) related to derivatives trading activities			$ 500	$ (2,800)		$ (4,900)	$ (10,400)

[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.