Investments of Operating Entities and Consolidated Funds - Derivatives (Details)			3 Months Ended		12 Months Ended
		Dec. 05, 2016	Mar. 31, 2018 USD ($) contract	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) contract	Dec. 14, 2017 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			$ 34,913,000			$ 69,177,000
Payable for derivative contracts, at fair value			48,875,000			42,750,000
Receivable on derivative contracts, at fair value, Consolidated Funds			1,866,000			2,520,000
Payable for derivative contracts, at fair value, Consolidated Funds			1,980,000			7,130,000
Equity Swap
Derivative [Line Items]
Receivable on derivative contracts, at fair value, Consolidated Funds			1,407,000			1,754,000
Payable for derivative contracts, at fair value, Consolidated Funds			1,660,000			7,042,000
Futures
Derivative [Line Items]
Derivative Asset, Notional Amount			3,978,850			3,819,350
Receivable on derivative contracts, at fair value			748,000			58,000
Derivative Liability, Notional Amount			26,583,000			16,796,000
Payable for derivative contracts, at fair value			209,000			242,000
Currency Forwards
Derivative [Line Items]
Derivative Asset, Notional Amount			1,775,435			233,136
Receivable on derivative contracts, at fair value			520,000			5,000
Derivative Liability, Notional Amount			77,431,000			62,439,000
Payable for derivative contracts, at fair value			620,000			874,000
Receivable on derivative contracts, at fair value, Consolidated Funds			4,000			524,000
Payable for derivative contracts, at fair value, Consolidated Funds			166,000			11,000
Swaps
Derivative [Line Items]
Derivative Asset, Notional Amount			64,488,819			164,664,203
Receivable on derivative contracts, at fair value			1,631,000			10,942,000
Derivative Liability, Notional Amount			27,927,000			11,595,000
Payable for derivative contracts, at fair value			$ 623,000			$ 71,000
Options
Derivative [Line Items]
Derivative Asset, Number of Instruments Held | contract	[1]		268,231			283,112
Receivable on derivative contracts, at fair value	[1]		$ 31,216,000			$ 58,172,000
Receivable on derivative contracts, at fair value, Consolidated Funds			455,000			242,000
Payable for derivative contracts, at fair value, Consolidated Funds			$ 154,000			$ 77,000
Put Option
Derivative [Line Items]
Derivative Liability, Number of Instruments Held | contract	[2]		63,328			57,043
Payable for derivative contracts, at fair value	[2]		$ 47,423,000			$ 41,563,000
Pay To Hold [Member]
Derivative [Line Items]
Derivative Asset, Number of Instruments Held | contract			0			0
Receivable on derivative contracts, at fair value			$ 798,000			$ 0
Receivable on derivatives contracts, at fair value [Member]
Derivative [Line Items]
Receivable on derivative contracts, at fair value			34,913,000			69,177,000
Derivative Asset, Fair Value, Gross Asset			34,913,000			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]		2,095,000			10,948,000
Derivative asset, net of offset			32,818,000			58,229,000
Payable for derivatives contracts, at fair value [Member]
Derivative [Line Items]
Payable for derivative contracts, at fair value			48,875,000			42,750,000
Financial Instruments, Owned and Pledged as Collateral, at Fair Value			0			0
Derivative Liability, Fair Value, Gross Liability			48,875,000			42,750,000
Derivative Liability, Fair Value, Amount Offset Against Collateral	[3]		0			0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	[4]		1,314,000			1,031,000
Derivative Liability, net of offset			47,561,000			41,719,000
Receivables from Brokers-Dealers and Clearing Organizations
Derivative [Line Items]
Collateral posted			11,900,000			$ 13,500,000
Other Income [Member]
Derivative [Line Items]
Realized and unrealized gains/(losses) related to derivatives trading activities			7,900,000	$ (8,600,000)
2022 convertible note [Member] | Convertible Debt
Derivative [Line Items]
Call Option, Fair Value			20,900,000				$ 23,400,000
2019 convertible note [Member] | Convertible Debt
Derivative [Line Items]
Call Option, Fair Value								$ 35,700,000
Open Option Contracts Written, at Fair Value			500,000
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.