Investments of Operating Entities and Consolidated Funds - Derivatives (Details)			12 Months Ended
		Dec. 05, 2016	Dec. 31, 2019 USD ($) contract	Dec. 31, 2018 USD ($) contract	Dec. 31, 2017 USD ($)
Derivative [Line Items]
Stockholders' Equity Note, Stock Split, Conversion Ratio		0.25
Receivable on derivative contracts, at fair value			$ 62,977,000	$ 25,125,000
Payable for derivative contracts, at fair value			60,761,000	16,082,000
Receivable on derivative contracts, at fair value			5,833,000	4,416,000
Payable for derivative contracts, at fair value, Consolidated Funds			4,769,000	1,663,000
Payables to brokers, dealers and clearing organizations			271,018,000	228,731,000
Receivable from brokers, dealers and clearing organizations, net of allowance of $721 and $472, respectively			681,695,000	786,113,000
Equity Swap
Derivative [Line Items]
Receivable on derivative contracts, at fair value			927,000	2,477,000
Payable for derivative contracts, at fair value, Consolidated Funds			3,931,000	713,000
Futures
Derivative [Line Items]
Derivative Asset, Notional Amount			0	42,288,000
Receivable on derivative contracts, at fair value			0	334,000
Derivative Liability, Notional Amount			10,224,000	0
Payable for derivative contracts, at fair value			217,000	0
Currency Forwards
Derivative [Line Items]
Derivative Asset, Notional Amount			0	395,000
Receivable on derivative contracts, at fair value			0	1,000
Derivative Liability, Notional Amount			77,790,000	96,406,000
Payable for derivative contracts, at fair value			851,000	709,000
Receivable on derivative contracts, at fair value			3,302,000	186,000
Payable for derivative contracts, at fair value, Consolidated Funds			88,000	96,000
Swaps
Derivative [Line Items]
Derivative Asset, Notional Amount			383,752,000	13,702,000
Receivable on derivative contracts, at fair value			2,911,000	917,000
Derivative Liability, Notional Amount			607,717,000	52,905,000
Payable for derivative contracts, at fair value			$ 23,169,000	$ 2,162,000
Options
Derivative [Line Items]
Derivative Asset, Number of Instruments Held | contract	[1]		550,188	654,506
Receivable on derivative contracts, at fair value	[1]		$ 60,066,000	$ 23,130,000
Receivable on derivative contracts, at fair value			1,604,000	1,753,000
Payable for derivative contracts, at fair value, Consolidated Funds			$ 750,000	$ 854,000
Option
Derivative [Line Items]
Derivative Liability, Number of Instruments Held | contract	[2]		306,306	90,730
Payable for derivative contracts, at fair value	[2]		$ 36,524,000	$ 13,211,000
Pay to hold
Derivative [Line Items]
Derivative Asset, Number of Instruments Held | contract			0	0
Receivable on derivative contracts, at fair value			$ 0	$ 743,000
Receivable on derivatives contracts, at fair value [Member]
Derivative [Line Items]
Receivable on derivative contracts, at fair value			62,977,000	25,125,000
Derivative Asset, Fair Value, Gross Asset			66,217,000	25,125,000
Derivative Asset, Fair Value, Amount Offset Against Collateral	[3]		3,240,000	0
Financial Instruments, Owned and Pledged as Collateral, at Fair Value			0	0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	[4]		2,911,000	1,662,000
Derivative asset, net of offset			60,066,000	23,463,000
Payable for derivatives contracts, at fair value [Member]
Derivative [Line Items]
Payable for derivative contracts, at fair value			60,761,000	16,082,000
Financial Instruments, Owned and Pledged as Collateral, at Fair Value			0	0
Derivative Liability, Fair Value, Gross Liability			64,001,000	16,082,000
Derivative Liability, Fair Value, Amount Offset Against Collateral	[3]		3,240,000	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	[4]		24,020,000	2,871,000
Derivative Liability, net of offset			36,741,000	13,211,000
Receivables from Brokers-Dealers and Clearing Organizations
Derivative [Line Items]
Collateral posted			10,500,000	11,200,000
Other Income [Member]
Derivative [Line Items]
Realized and unrealized gains/(losses) related to derivatives trading activities			$ (13,000,000.0)	$ 14,300,000	$ 3,900,000

[1] Includes the volume of contracts for index, equity, commodity future and cash conversion options.

[2] Includes the volume of contracts for 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.