Investments of Operating Entities and Consolidated Funds - Derivatives (Details)			3 Months Ended		9 Months Ended
		Dec. 05, 2016	Sep. 30, 2019 USD ($) contract	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) contract	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) contract
Derivative [Line Items]
Stockholders' Equity Note, Stock Split, Conversion Ratio		0.25
Receivable on derivative contracts, at fair value			$ 39,754,000		$ 39,754,000		$ 25,125,000
Payable for derivative contracts, at fair value			32,755,000		32,755,000		16,082,000
Receivable on derivative contracts, at fair value			3,885,000		3,885,000		4,416,000
Payable for derivative contracts, at fair value, Consolidated Funds			7,778,000		7,778,000		1,663,000
Equity Swap
Derivative [Line Items]
Receivable on derivative contracts, at fair value			1,296,000		1,296,000		2,477,000
Payable for derivative contracts, at fair value, Consolidated Funds			5,022,000		5,022,000		713,000
Futures
Derivative [Line Items]
Derivative Asset, Notional Amount			33,473,750		33,473,750		42,288,209
Receivable on derivative contracts, at fair value			103,000		103,000		334,000
Currency Forwards
Derivative [Line Items]
Derivative Asset, Notional Amount			79,804,033		79,804,033		394,875
Receivable on derivative contracts, at fair value			640,000		640,000		1,000
Derivative Liability, Notional Amount			24,636,000		24,636,000		96,406,000
Payable for derivative contracts, at fair value			91,000		91,000		709,000
Receivable on derivative contracts, at fair value			31,000		31,000		186,000
Payable for derivative contracts, at fair value, Consolidated Funds			2,341,000		2,341,000		96,000
Swaps
Derivative [Line Items]
Derivative Asset, Notional Amount			103,591,441		103,591,441		13,702,458
Receivable on derivative contracts, at fair value			4,242,000		4,242,000		917,000
Derivative Liability, Notional Amount			164,056,000		164,056,000		52,905,000
Payable for derivative contracts, at fair value			$ 6,502,000		$ 6,502,000		$ 2,162,000
Options
Derivative [Line Items]
Derivative Asset, Number of Instruments Held | contract	[1]		349,434		349,434		654,506
Receivable on derivative contracts, at fair value	[1]		$ 34,769,000		$ 34,769,000		$ 23,130,000
Receivable on derivative contracts, at fair value			2,558,000		2,558,000		1,753,000
Payable for derivative contracts, at fair value, Consolidated Funds			$ 415,000		$ 415,000		$ 854,000
Option
Derivative [Line Items]
Derivative Liability, Number of Instruments Held | contract	[2]		190,321		190,321		90,730
Payable for derivative contracts, at fair value	[2]		$ 26,162,000		$ 26,162,000		$ 13,211,000
Pay to hold
Derivative [Line Items]
Derivative Asset, Number of Instruments Held | contract			0		0		0
Receivable on derivative contracts, at fair value			$ 0		$ 0		$ 743,000
Receivable on derivatives contracts, at fair value [Member]
Derivative [Line Items]
Receivable on derivative contracts, at fair value			39,754,000		39,754,000		25,125,000
Derivative Asset, Fair Value, Gross Asset			39,754,000		39,754,000		25,125,000
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]		4,882,000		4,882,000		1,662,000
Derivative asset, net of offset			34,872,000		34,872,000		23,463,000
Payable for derivatives contracts, at fair value [Member]
Derivative [Line Items]
Payable for derivative contracts, at fair value			32,755,000		32,755,000		16,082,000
Financial Instruments, Owned and Pledged as Collateral, at Fair Value			0		0		0
Derivative Liability, Fair Value, Gross Liability			32,755,000		32,755,000		16,082,000
Derivative Liability, Fair Value, Amount Offset Against Collateral	[3]		0		0		0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	[4]		6,592,000		6,592,000		2,871,000
Derivative Liability, net of offset			26,163,000		26,163,000		13,211,000
Receivables from Brokers-Dealers and Clearing Organizations
Derivative [Line Items]
Collateral posted			8,200,000		8,200,000		$ 11,200,000
Other Income [Member]
Derivative [Line Items]
Realized and unrealized gains/(losses) related to derivatives trading activities			$ 7,100,000	$ (7,500,000)	$ 2,300,000	$ 6,400,000

[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.