Investments of Operating Entities and Consolidated Funds - Derivatives (Details)			3 Months Ended		9 Months Ended		12 Months Ended
		Dec. 05, 2016	Sep. 30, 2018 USD ($) contract	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) contract	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) contract	Mar. 10, 2014 USD ($)
Derivative [Line Items]
Warrants issued							$ 15,200,000
Cost of hedge transaction and warrant, net									$ 20,500,000
Stockholders' Equity Note, Stock Split, Conversion Ratio		0.25
Receivable on derivative contracts, at fair value			$ 32,229,000		$ 32,229,000			$ 69,177,000
Payable for derivative contracts, at fair value			30,002,000		30,002,000			42,750,000
Receivable on derivative contracts, at fair value			1,911,000		1,911,000			2,520,000
Payable for derivative contracts, at fair value, Consolidated Funds			2,189,000		2,189,000			7,130,000
Equity Swap
Derivative [Line Items]
Receivable on derivative contracts, at fair value			1,584,000		1,584,000			1,754,000
Payable for derivative contracts, at fair value, Consolidated Funds			1,590,000		1,590,000			7,042,000
Futures
Derivative [Line Items]
Derivative Asset, Notional Amount			0		0			3,819,350
Receivable on derivative contracts, at fair value			0		0			58,000
Derivative Liability, Notional Amount			25,474,000		25,474,000			16,796,000
Payable for derivative contracts, at fair value			71,000		71,000			242,000
Currency Forwards
Derivative [Line Items]
Derivative Asset, Notional Amount			79,309,515		79,309,515			233,136
Receivable on derivative contracts, at fair value			1,088,000		1,088,000			5,000
Derivative Liability, Notional Amount			2,897,000		2,897,000			62,439,000
Payable for derivative contracts, at fair value			4,000		4,000			874,000
Receivable on derivative contracts, at fair value			27,000		27,000			524,000
Payable for derivative contracts, at fair value, Consolidated Funds			539,000		539,000			11,000
Swaps
Derivative [Line Items]
Derivative Asset, Notional Amount			57,925,887		57,925,887			164,664,203
Receivable on derivative contracts, at fair value			1,203,000		1,203,000			10,942,000
Derivative Liability, Notional Amount			77,775,000		77,775,000			11,595,000
Payable for derivative contracts, at fair value			$ 2,743,000		$ 2,743,000			$ 71,000
Options
Derivative [Line Items]
Derivative Asset, Number of Instruments Held | contract	[1]		660,776		660,776			283,112
Receivable on derivative contracts, at fair value	[1]		$ 29,330,000		$ 29,330,000			$ 58,172,000
Receivable on derivative contracts, at fair value			300,000		300,000			242,000
Payable for derivative contracts, at fair value, Consolidated Funds			$ 60,000		$ 60,000			$ 77,000
Option
Derivative [Line Items]
Derivative Liability, Number of Instruments Held | contract	[2]		96,500		96,500			57,043
Payable for derivative contracts, at fair value	[2]		$ 27,184,000		$ 27,184,000			$ 41,563,000
Pay to hold
Derivative [Line Items]
Derivative Asset, Number of Instruments Held | contract			0		0			0
Receivable on derivative contracts, at fair value			$ 608,000		$ 608,000			$ 0
Receivable on derivatives contracts, at fair value [Member]
Derivative [Line Items]
Receivable on derivative contracts, at fair value			32,229,000		32,229,000			69,177,000
Derivative Asset, Fair Value, Gross Asset			32,229,000		32,229,000			69,177,000
Derivative Asset, Fair Value, Amount Offset Against Collateral	[3]		0		0			0
Financial Instruments, Owned and Pledged as Collateral, at Fair Value			0		0			0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	[4]		2,899,000		2,899,000			10,948,000
Derivative asset, net of offset			29,330,000		29,330,000			58,229,000
Payable for derivatives contracts, at fair value [Member]
Derivative [Line Items]
Payable for derivative contracts, at fair value			30,002,000		30,002,000			42,750,000
Financial Instruments, Owned and Pledged as Collateral, at Fair Value			0		0			0
Derivative Liability, Fair Value, Gross Liability			30,002,000		30,002,000			42,750,000
Derivative Liability, Fair Value, Amount Offset Against Collateral	[3]		0		0			0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	[4]		2,747,000		2,747,000			1,031,000
Derivative Liability, net of offset			27,255,000		27,255,000			41,719,000
Receivables from Brokers-Dealers and Clearing Organizations
Derivative [Line Items]
Collateral posted			9,800,000		9,800,000			$ 13,500,000
Other Income [Member]
Derivative [Line Items]
Realized and unrealized gains/(losses) related to derivatives trading activities			(7,500,000)	$ 500,000	6,400,000	$ (4,900,000)
2019 convertible note | Convertible Debt
Derivative [Line Items]
Call Option, Fair Value									$ 35,700,000
Open Option Contracts Written, at Fair Value			200,000		$ 200,000
Derivative, Loss on Derivative			$ 600,000

[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.