Investments of Operating Entities and Consolidated Funds - Derivatives (Details) $ / shares in Units, $ in Thousands			12 Months Ended
		Dec. 05, 2016	Dec. 31, 2017 USD ($) contract shares	Dec. 31, 2016 USD ($) contract	Dec. 31, 2015 USD ($)	Dec. 14, 2017 USD ($)	Mar. 10, 2014 USD ($) $ / shares
Derivative [Line Items]
Warrants issued					$ 15,200
Cost of hedge transaction and warrant, net							$ 20,500
Stockholders' Equity Note, Stock Split, Conversion Ratio		0.25
Trading days for expiration			80 days
Receivable on derivative contracts, at fair value			$ 69,177	$ 22,901
Payable for derivative contracts, at fair value			42,750	20,762
Receivable on derivatives, Consolidated Funds			2,520	893
Payable for derivative contracts, at fair value, Consolidated Funds			7,130	572
Currency forward
Derivative [Line Items]
Receivable on derivatives, Consolidated Funds			524	18
Payable for derivative contracts, at fair value, Consolidated Funds			11	10
Equity Swap
Derivative [Line Items]
Receivable on derivatives, Consolidated Funds			1,754	731
Payable for derivative contracts, at fair value, Consolidated Funds			7,042	495
Futures
Derivative [Line Items]
Derivative Asset, Notional Amount			3,819	12,421
Receivable on derivative contracts, at fair value			58	104
Derivative Liability, Notional Amount			16,796	38,345
Payable for derivative contracts, at fair value			242	642
Currency Forwards
Derivative [Line Items]
Derivative Asset, Notional Amount			233	80,608
Receivable on derivative contracts, at fair value			5	592
Derivative Liability, Notional Amount			62,439	0
Payable for derivative contracts, at fair value			874	0
Swaps
Derivative [Line Items]
Derivative Asset, Notional Amount			164,664	46,462
Receivable on derivative contracts, at fair value			10,942	468
Derivative Liability, Notional Amount			11,595	9,533
Payable for derivative contracts, at fair value			$ 71	$ 181
Options
Derivative [Line Items]
Derivative Asset, Number of Instruments Held | contract	[1]		283,112	256,097
Receivable on derivative contracts, at fair value	[1]		$ 58,172	$ 21,539
Receivable on derivatives, Consolidated Funds			242	144
Payable for derivative contracts, at fair value, Consolidated Funds			77	67
Foreign Exchange Option
Derivative [Line Items]
Derivative Asset, Notional Amount			0	57,051
Receivable on derivative contracts, at fair value			$ 0	$ 198
Put Option
Derivative [Line Items]
Derivative Liability, Number of Instruments Held | contract	[2]		57,043	23,726
Payable for derivative contracts, at fair value	[2]		$ 41,563	$ 19,939
Receivable on derivatives contracts, at fair value [Member]
Derivative [Line Items]
Receivable on derivative contracts, at fair value			69,177	22,901
Derivative Asset, Fair Value, Gross Asset			69,177	22,901
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]		10,948	1,382
Derivative asset, net of offset			58,229	21,519
Payable for derivatives contracts, at fair value [Member]
Derivative [Line Items]
Payable for derivative contracts, at fair value			42,750	20,762
Financial Instruments, Owned and Pledged as Collateral, at Fair Value			0	0
Derivative Liability, Fair Value, Gross Liability			42,750	20,762
Derivative Liability, Fair Value, Amount Offset Against Collateral	[3]		0	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	[4]		1,031	181
Derivative Liability, net of offset			41,719	20,581
Receivables from Brokers-Dealers and Clearing Organizations
Derivative [Line Items]
Collateral posted			$ 13,500	17,100
Common Stock Class A
Derivative [Line Items]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights | shares			7,012,196
Class of Warrant, Exercise Price of Warrants or Rights | $ / shares							$ 28.72
Other Income [Member]
Derivative [Line Items]
Realized and unrealized gains/(losses) related to derivatives trading activities			$ 3,900	$ (11,200)	$ (5,600)
2022 convertible note [Member] | Convertible Debt
Derivative [Line Items]
Call Option, Fair Value			22,100			$ 23,400
2019 convertible note [Member] | Convertible Debt
Derivative [Line Items]
Call Option, Fair Value			1,500				$ 35,700
Open Option Contracts Written, at Fair Value			$ 300
Class of Warrant, Exercise Price of Warrants or Rights | $ / shares							$ 28.72

[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.