Investments of Operating Entities and Consolidated Funds - Derivatives (Details)			12 Months Ended
		Dec. 05, 2016	Dec. 31, 2018 USD ($) contract	Dec. 31, 2017 USD ($) contract	Dec. 31, 2016 USD ($)	Mar. 10, 2014 USD ($)
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			$ 25,125,000	$ 69,177,000
Payable for derivative contracts, at fair value			16,082,000	42,750,000
Receivable on derivative contracts, at fair value			4,416,000	2,520,000
Payable for derivative contracts, at fair value, Consolidated Funds			1,663,000	7,130,000
Equity Swap
Derivative [Line Items]
Receivable on derivative contracts, at fair value			2,477,000	1,754,000
Payable for derivative contracts, at fair value, Consolidated Funds			713,000	7,042,000
Futures
Derivative [Line Items]
Derivative Asset, Notional Amount			42,288,209	3,819,350
Receivable on derivative contracts, at fair value			334,000	58,000
Derivative Liability, Notional Amount			0	16,796,000
Payable for derivative contracts, at fair value			0	242,000
Currency Forwards
Derivative [Line Items]
Derivative Asset, Notional Amount			394,875	233,136
Receivable on derivative contracts, at fair value			1,000	5,000
Derivative Liability, Notional Amount			96,406,000	62,439,000
Payable for derivative contracts, at fair value			709,000	874,000
Receivable on derivative contracts, at fair value			186,000	524,000
Payable for derivative contracts, at fair value, Consolidated Funds			96,000	11,000
Swaps
Derivative [Line Items]
Derivative Asset, Notional Amount			13,702,458	164,664,203
Receivable on derivative contracts, at fair value			917,000	10,942,000
Derivative Liability, Notional Amount			52,905,000	11,595,000
Payable for derivative contracts, at fair value			$ 2,162,000	$ 71,000
Options
Derivative [Line Items]
Derivative Asset, Number of Instruments Held | contract	[1]		654,506	283,112
Receivable on derivative contracts, at fair value	[1]		$ 23,130,000	$ 58,172,000
Receivable on derivative contracts, at fair value			1,753,000	242,000
Payable for derivative contracts, at fair value, Consolidated Funds			$ 854,000	$ 77,000
Option
Derivative [Line Items]
Derivative Liability, Number of Instruments Held | contract	[2]		90,730	57,043
Payable for derivative contracts, at fair value	[2]		$ 13,211,000	$ 41,563,000
Pay to hold
Derivative [Line Items]
Derivative Asset, Number of Instruments Held | contract			0	0
Receivable on derivative contracts, at fair value			$ 743,000	$ 0
Receivable on derivatives contracts, at fair value [Member]
Derivative [Line Items]
Receivable on derivative contracts, at fair value			25,125,000	69,177,000
Derivative Asset, Fair Value, Gross Asset			25,125,000	69,177,000
Derivative Asset, Fair Value, Amount Offset Against Collateral	[3]		0	0
Financial Instruments, Owned and Pledged as Collateral, at Fair Value			0	0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	[4]		1,662,000	10,948,000
Derivative asset, net of offset			23,463,000	58,229,000
Payable for derivatives contracts, at fair value [Member]
Derivative [Line Items]
Payable for derivative contracts, at fair value			16,082,000	42,750,000
Financial Instruments, Owned and Pledged as Collateral, at Fair Value			0	0
Derivative Liability, Fair Value, Gross Liability			16,082,000	42,750,000
Derivative Liability, Fair Value, Amount Offset Against Collateral	[3]		0	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	[4]		2,871,000	1,031,000
Derivative Liability, net of offset			13,211,000	41,719,000
Receivables from Brokers-Dealers and Clearing Organizations
Derivative [Line Items]
Collateral posted			11,200,000	13,500,000
Other Income [Member]
Derivative [Line Items]
Realized and unrealized gains/(losses) related to derivatives trading activities			14,300,000	$ 3,900,000	$ (11,200,000)
2019 convertible note | Convertible Debt
Derivative [Line Items]
Call Option, Fair Value						$ 35,700,000
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.