Investments of Operating Entities and Consolidated Funds - Derivatives (Details) $ / shares in Units, $ in Thousands			12 Months Ended
		Dec. 05, 2016	Dec. 31, 2016 USD ($) contract shares	Dec. 31, 2015 USD ($) contract	Dec. 31, 2014 USD ($)	Mar. 10, 2014 USD ($) $ / shares
Derivative [Line Items]
Payable for derivatives, Consolidated Funds			$ 572	$ 0
Receivable on derivatives, Consolidated Funds			$ 893	0
Stockholders' Equity Note, Stock Split, Conversion Ratio		0.25
Warrants issued					$ 15,218
Cost of hedge transaction and warrant, net						$ 20,500
Trading days for expiration			80 days
Receivable on derivative contracts, at fair value			$ 22,901	39,618
Payable for derivative contracts, at fair value			20,762	21,183
Currency forward
Derivative [Line Items]
Payable for derivatives, Consolidated Funds			10
Receivable on derivatives, Consolidated Funds			18
Futures
Derivative [Line Items]
Payable for derivatives, Consolidated Funds			495
Receivable on derivative contracts, at fair value			104	189
Payable for derivative contracts, at fair value			642	101
Derivative Asset, Notional Amount			12,421	9,416
Derivative Liability, Notional Amount			38,345	11,995
Currency Forwards
Derivative [Line Items]
Receivable on derivative contracts, at fair value			592	659
Payable for derivative contracts, at fair value			0	463
Derivative Asset, Notional Amount			80,608	67,862
Derivative Liability, Notional Amount			0	44,156
Equity Swaps
Derivative [Line Items]
Receivable on derivatives, Consolidated Funds			731
Receivable on derivative contracts, at fair value			468	2,327
Payable for derivative contracts, at fair value			181	71
Derivative Asset, Notional Amount			46,462	118,488
Derivative Liability, Notional Amount			9,533	7,605
Options
Derivative [Line Items]
Payable for derivatives, Consolidated Funds			67
Receivable on derivatives, Consolidated Funds			144
Receivable on derivative contracts, at fair value	[1]		$ 21,539	$ 31,456
Derivative Asset, Number of Instruments Held | contract	[1]		256,097	289,433
Put Option
Derivative [Line Items]
Payable for derivative contracts, at fair value	[2]		$ 19,939	$ 20,548
Derivative Liability, Number of Instruments Held | contract	[2]		23,726	16,632
Foreign Exchange Option
Derivative [Line Items]
Receivable on derivative contracts, at fair value			$ 198	$ 4,987
Derivative Asset, Notional Amount			57,051	283,797
Receivables from Brokers-Dealers and Clearing Organizations
Derivative [Line Items]
Collateral posted			17,100	27,100
Receivable on derivatives contracts, at fair value [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset			22,901	39,618
Derivative Asset, Fair Value, Amount Offset Against Collateral	[3]		0	0
Receivable on derivative contracts, at fair value			22,901	39,618
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	[4]		1,382	9,339
Derivative asset, net of offset			21,519	30,279
Financial Instruments, Owned and Pledged as Collateral, at Fair Value			0	0
Payable for derivatives contracts, at fair value [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability			20,762	21,183
Derivative Liability, Fair Value, Amount Offset Against Collateral	[3]		0	0
Payable for derivative contracts, at fair value			20,762	21,183
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	[4]		181	534
Derivative Liability, net of offset			20,581	20,649
Financial Instruments, Owned and Pledged as Collateral, at Fair Value			0	0
Enterprise Master
Derivative [Line Items]
Receivable (payable) on derivative contracts, at fair value			0	(4)
Merger Master
Derivative [Line Items]
Receivable on derivative contracts, at fair value			4,519	2,511
Payable for derivative contracts, at fair value			2,408	593
Merger Master | Currency Forwards
Derivative [Line Items]
Receivable on derivative contracts, at fair value			0	235
Merger Master | Equity Swaps
Derivative [Line Items]
Receivable on derivative contracts, at fair value			255	1,001
Payable for derivative contracts, at fair value			123	30
Merger Master | Options
Derivative [Line Items]
Receivable on derivative contracts, at fair value			4,264	1,275
Merger Master | Put Option
Derivative [Line Items]
Payable for derivative contracts, at fair value			$ 2,285	563
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
Convertible Debt
Derivative [Line Items]
Call Option, Fair Value			$ 14,800			$ 35,700
Other Income [Member]
Derivative [Line Items]
Realized and unrealized gains/(losses) related to derivatives trading activities			(11,200)	(5,600)	$ (500)
Currency Forwards | Enterprise Master
Derivative [Line Items]
Receivable (payable) on derivative contracts, at fair value			$ 0	$ (4)

[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.