Fair Value Measurements for Operating Entities and Consolidated Funds Unobservable Input Roll Forward (Details) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013		Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, Between Level 1 and 2 transfers, amount	$ 0				$ 0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	244,474,000				244,474,000
Balance Liability Value	35,204,000				35,204,000
Preferred Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	574,000		333,000		574,000		333,000		2,574,000	324,000	333,000	2,332,000
Asset, Transfers In	0		0		0		0
Asset, Transfers Out	(2,000,000)	[1]	0		(2,000,000)	[1]	(2,000,000)	[1]
Purchases/(covers)	0		0		2,250,000		0
(Sales)/short buys	0		0		0		0
Realized and unrealized gains (losses), asset	0		0		0		1,000
Change in Unrealized Gain (Loss), instruments still held, asset	0	[2]	0	[2]	0	[2]	1,000	[2]
Common Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	290,000		2,109,000		290,000		2,109,000		419,000	3,559,000	2,273,000	2,549,000
Asset, Transfers In	0		0		0		0
Asset, Transfers Out	0		0		(3,150,000)	[1]	0
Purchases/(covers)	0		0		9,000		0
(Sales)/short buys	(1,000)		(2,000)		(1,000)		(275,000)
Realized and unrealized gains (losses), asset	(128,000)		(162,000)		(127,000)		(165,000)
Change in Unrealized Gain (Loss), instruments still held, asset	(14,000)	[2]	(161,000)	[2]	(13,000)	[2]	(424,000)	[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	28,785,000				28,785,000				37,658,000	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	(8,873,000)				(6,925,000)
Change in Unrealized Gain (Loss), instruments still held, asset	(8,873,000)	[2]			(6,925,000)	[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				1,750,000	1,950,000
Asset, Transfers In	0				0
Asset, Transfers Out	0				0
Purchases/(covers)	0				0
(Sales)/short buys	0				(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	2,608,000		2,667,000		2,608,000		2,667,000		3,346,000	5,805,000	2,136,000	1,713,000
Asset, Transfers In	0		0		0		290,000	[3]
Asset, Transfers Out	0		0		0		0
Purchases/(covers)	57,000		0		57,000		166,000
(Sales)/short buys	(57,000)		0		(1,385,000)		(110,000)
Realized and unrealized gains (losses), asset	(738,000)		531,000		(1,869,000)		608,000
Change in Unrealized Gain (Loss), instruments still held, asset	(434,000)	[2]	(76,000)	[2]	(432,000)	[2]	760,000	[2]
Portfolio Funds, at fair value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	60,474,000		51,703,000		60,474,000		51,703,000		62,437,000	51,649,000	43,343,000	25,670,000
Asset, Transfers In	0				0		13,128,000
Asset, Transfers Out	0		0		0		0
Purchases/(covers)	3,049,000		6,491,000		12,234,000		16,560,000
(Sales)/short buys	(5,320,000)		(289,000)		(8,340,000)		(7,821,000)
Realized and unrealized gains (losses), asset	308,000		2,158,000		4,931,000		4,166,000
Change in Unrealized Gain (Loss), instruments still held, asset	373,000	[2]	1,446,000	[2]	3,585,000	[2]	4,542,000	[2]
Real Estate Investments, at fair value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	2,285,000		2,127,000		2,285,000		2,127,000		2,262,000	2,088,000	2,158,000	1,864,000
Asset, Transfers In	0		0		0		0
Asset, Transfers Out	0		0		0		0
Purchases/(covers)	0		0		50,000,000		0
(Sales)/short buys	0		0		(50,027,000)		0
Realized and unrealized gains (losses), asset	23,000		(31,000)		224,000		263,000
Change in Unrealized Gain (Loss), instruments still held, asset	23,000	[2]	(31,000)	[2]	224,000	[2]	263,000	[2]
Lehman claims, at fair value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	360,000		558,000		360,000		558,000		418,000	378,000	278,000	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	0		0		(76,000)		(382,000)
Realized and unrealized gains (losses), asset	(58,000)		280,000		58,000		234,000
Change in Unrealized Gain (Loss), instruments still held, asset	(58,000)	[2]	280,000	[2]	58,000	[2]	234,000	[2]
Consolidated Funds | Portfolio Funds, at fair value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	146,987,000		161,086,000		146,987,000		161,086,000		145,371,000	155,674,000	177,848,000	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	(409,000)		(17,886,000)		(16,439,000)		(27,835,000)
Realized and unrealized gains (losses), asset	2,025,000		1,124,000		7,752,000		6,001,000
Change in Unrealized Gain (Loss), instruments still held, asset	1,962,000	[2]	635,000	[2]	7,689,000	[2]	5,092,000	[2]
Consolidated Funds | Lehman claims, at fair value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	361,000		16,483,000		361,000		16,483,000		4,090,000	4,842,000	15,434,000	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	(3,731,000)		0		(4,711,000)		(1,449,000)
Realized and unrealized gains (losses), asset	2,000		1,049,000		230,000		3,808,000
Change in Unrealized Gain (Loss), instruments still held, asset	2,000	[2]	1,049,000	[2]	230,000	[2]	2,448,000	[2]
Options, liability
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Liability Value	28,785,000				28,785,000				37,658,000	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	(8,873,000)				(6,925,000)
Change in Unrealized Gain (Loss), instruments still held, liabilities	(8,873,000)	[2]			(6,925,000)	[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
Liability, Transfers In							0
Liability, Transfers Out							0
Liability, Purchases							0
Liability, Sales							0
Realized and unrealized gains (losses), liability							(3,000)
Change in Unrealized Gain (Loss), instruments still held, liabilities							0	[2]
Contingent liability payable
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Liability Value	6,419,000		7,834,000		6,419,000		7,834,000		6,783,000	6,937,000	7,928,000	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	(364,000)		(94,000)		(518,000)		(282,000)
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	1,500,000				1,500,000					2,100,000
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High	$ 10,900,000				$ 10,900,000					$ 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).