Fair Value Measurements for Operating Entities and Consolidated Funds Unobservable Input Roll Forward (Details) (USD $)	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013		Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	$ 260,325,000				$ 260,325,000
Balance Liability Value	44,441,000				44,441,000
Preferred Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	2,324,000		333,000		2,324,000		333,000		324,000	324,000	2,332,000	2,332,000
Asset, Transfers In	0		0		0		0
Asset, Transfers Out	0		(2,000,000)	[1]	0		(2,000,000)	[1]
Purchases/(covers)	2,000,000		0		2,000,000		0
(Sales)/short buys	0		0		0		0
Realized and unrealized gains (losses), asset	0		1,000		0		1,000
Change in Unrealized Gain (Loss), instruments still held, asset	0	[2]	1,000	[2]	0	[2]	1,000	[2]
Common Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	419,000		2,273,000		419,000		2,273,000		410,000	3,559,000	2,278,000	2,549,000
Asset, Transfers In	0		0		0		0
Asset, Transfers Out	0		0		(3,150,000)	[1]	0
Purchases/(covers)	9,000		0		9,000		0
(Sales)/short buys	0		(8,000)		0		(273,000)
Realized and unrealized gains (losses), asset	0		3,000		1,000		(3,000)
Change in Unrealized Gain (Loss), instruments still held, asset	0	[2]	2,000	[2]	1,000	[2]	(263,000)	[2]
Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value			0				0					515,000
Asset, Transfers In							0
Asset, Transfers Out							0
Purchases/(covers)							2,735,000
(Sales)/short buys							(3,346,000)
Realized and unrealized gains (losses), asset							96,000
Change in Unrealized Gain (Loss), instruments still held, asset							0	[2]
Options, asset
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	37,658,000				37,658,000				42,856,000	0
Asset, Transfers In	0				0
Asset, Transfers Out	0				0
Purchases/(covers)	0				35,710,000
(Sales)/short buys	0				0
Realized and unrealized gains (losses), asset	(5,198,000)				1,948,000
Change in Unrealized Gain (Loss), instruments still held, asset	(5,198,000)	[2]			1,948,000	[2]
Convertible Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	1,750,000				1,750,000				1,950,000	1,950,000
Asset, Transfers In	0				0
Asset, Transfers Out	0				0
Purchases/(covers)	0				0
(Sales)/short buys	(200,000)				(200,000)
Realized and unrealized gains (losses), asset	0				0
Change in Unrealized Gain (Loss), instruments still held, asset	0	[2]			0	[2]
Warrants and Rights
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	3,346,000		2,136,000		3,346,000		2,136,000		3,268,000	5,805,000	3,243,000	1,713,000
Asset, Transfers In	0		0	[3]	0		290,000	[3]
Asset, Transfers Out	0		0		0		0
Purchases/(covers)	0		0		0		166,000
(Sales)/short buys	0		0		(1,328,000)		(110,000)
Realized and unrealized gains (losses), asset	78,000		(1,107,000)		(1,131,000)		77,000
Change in Unrealized Gain (Loss), instruments still held, asset	108,000	[2]	(998,000)	[2]	244,000	[2]	270,000	[2]
Portfolio Funds, at fair value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	62,687,000		32,832,000		62,687,000		32,832,000		58,583,000	51,649,000	25,559,000	25,670,000
Asset, Transfers In	0		0		0		0
Asset, Transfers Out	0		0		0		0
Purchases/(covers)	5,050,000		8,981,000		9,435,000		12,571,000
(Sales)/short buys	(1,778,000)		(2,857,000)		(3,020,000)		(6,733,000)
Realized and unrealized gains (losses), asset	832,000		1,149,000		4,623,000		1,324,000
Change in Unrealized Gain (Loss), instruments still held, asset	(187,000)	[2]	1,437,000	[2]	3,212,000	[2]	1,604,000	[2]
Real Estate Investments, at fair value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	2,262,000		2,158,000		2,262,000		2,158,000		42,283,000	2,088,000	1,875,000	1,864,000
Asset, Transfers In	0		0		0		0
Asset, Transfers Out	0		0		0		0
Purchases/(covers)	10,000,000		0		50,000,000		0
(Sales)/short buys	(50,000,000)		0		(50,027,000)		0
Realized and unrealized gains (losses), asset	(21,000)		283,000		201,000		294,000
Change in Unrealized Gain (Loss), instruments still held, asset	(21,000)	[2]	283,000	[2]	201,000	[2]	294,000	[2]
Lehman claims, at fair value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	418,000		278,000		418,000		278,000		390,000	378,000	660,000	706,000
Asset, Transfers In	0		0		0		0
Asset, Transfers Out	0		0		0		0
Purchases/(covers)	0		0		0		0
(Sales)/short buys	(76,000)		(382,000)		(76,000)		(382,000)
Realized and unrealized gains (losses), asset	104,000		0		116,000		(46,000)
Change in Unrealized Gain (Loss), instruments still held, asset	104,000	[2]	0	[2]	116,000	[2]	(46,000)	[2]
Consolidated Funds | Portfolio Funds, at fair value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	145,371,000		177,848,000		145,371,000		177,848,000		143,059,000	155,674,000	178,357,000	182,920,000
Asset, Transfers In	0		0		0		0
Asset, Transfers Out	0		0		0		0
Purchases/(covers)	0		0		0		0
(Sales)/short buys	(86,000)		(2,918,000)		(16,030,000)		(9,949,000)
Realized and unrealized gains (losses), asset	2,398,000		2,429,000		5,727,000		4,877,000
Change in Unrealized Gain (Loss), instruments still held, asset	2,398,000	[2]	2,312,000	[2]	5,727,000	[2]	4,557,000	[2]
Consolidated Funds | Lehman claims, at fair value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	4,090,000		15,434,000		4,090,000		15,434,000		3,776,000	4,842,000	15,140,000	14,124,000
Asset, Transfers In	0		0		0		0
Asset, Transfers Out	0		0		0		0
Purchases/(covers)	0		0		0		0
(Sales)/short buys	0		0		(980,000)		(1,449,000)
Realized and unrealized gains (losses), asset	314,000		294,000		228,000		2,759,000
Change in Unrealized Gain (Loss), instruments still held, asset	314,000	[2]	293,000	[2]	228,000	[2]	1,399,000	[2]
Options, liability
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Liability Value	37,658,000				37,658,000				42,856,000	0
Liability, Transfers In	0				0
Liability, Transfers Out	0				0
Liability, Purchases	0				(35,710,000)
Liability, Sales	0				0
Realized and unrealized gains (losses), liability	(5,198,000)				1,948,000
Change in Unrealized Gain (Loss), instruments still held, liabilities	(5,198,000)	[2]			1,948,000	[2]
Warrants and Rights, sold not yet purchased
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Liability Value			0				0				3,000	3,000
Liability, Transfers In			0				0
Liability, Transfers Out			0				0
Liability, Purchases			0				0
Liability, Sales			0				0
Realized and unrealized gains (losses), liability			3,000				3,000
Change in Unrealized Gain (Loss), instruments still held, liabilities			0	[2]			0	[2]
Contingent liability payable
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Liability Value	6,783,000		7,928,000		6,783,000		7,928,000		6,861,000	6,937,000	8,116,000	8,116,000
Liability, Transfers In	0		0		0		0
Liability, Transfers Out	0		0		0		0
Liability, Purchases	0		0		0		0
Liability, Sales	(78,000)		(188,000)		(154,000)		(188,000)
Realized and unrealized gains (losses), liability	0		0		0		0
Change in Unrealized Gain (Loss), instruments still held, liabilities	0	[2]	0	[2]	0	[2]	0	[2]
ATM Group and Cowen Equity Finance LP [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, Low	1,600,000				1,600,000					2,100,000
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High	$ 12,900,000				$ 12,900,000					$ 13,800,000

[1] The company completed an initial public offering.

[2] Unrealized gains/losses are reported in other income (loss) in the accompanying condensed consolidated statements of operations.

[3] The security was acquired through an acquisition (See Note 2).