Fair Value Measurements for Operating Entities and Consolidated Funds Unobservable Input Roll Forward (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Securities sold, not yet purchased, at fair value, end of period	$ 177,937		$ 334,251
Level 3 | Operating Companies
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Investments at fair value, end of period	7,812
Fair Value, Measurements, Recurring | Level 3 | Operating Companies | Common Stock
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Securities sold, not yet purchased, at fair value, end of period	0		0
Fair Value, Measurements, Recurring | Level 3 | Operating Companies | Warrants and Rights
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Securities sold, not yet purchased, at fair value, end of period	3
Fair Value, Measurements, Recurring | Level 3 | Operating Companies | Common stock, sold not yet purchased
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Investments at fair value, beginning of period	0		0
Transfers in	0		0
Transfers out	0		0
Purchases/(covers)	0		(978)
(Sales)/short buys	0		833
Realized gains (losses)	0		145
Unrealized gains (losses)	0		0
Investments at fair value, end of period	0		0
Fair Value, Measurements, Recurring | Level 3 | Operating Companies | Preferred Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Investments at fair value, beginning of period	250		0
Transfers in	0		0
Transfers out	0		0
Purchases/(covers)	2,000		250
(Sales)/short buys	0		0
Realized gains (losses)	0		0
Unrealized gains (losses)	82		0
Investments at fair value, end of period	2,332		250
Fair Value, Measurements, Recurring | Level 3 | Operating Companies | Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Investments at fair value, beginning of period	0		0
Transfers in	0		0
Transfers out	0		0
Purchases/(covers)	4,600		0
(Sales)/short buys	(3,050)		0
Realized gains (losses)	0		0
Unrealized gains (losses)	(1,035)		0
Investments at fair value, end of period	515		0
Fair Value, Measurements, Recurring | Level 3 | Operating Companies | Warrant
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Investments at fair value, beginning of period	1,534		1,977
Transfers in			0
Transfers out	(89)	[1]	0
Purchases/(covers)	632		111
(Sales)/short buys	(212)		(84)
Realized gains (losses)	56		48
Unrealized gains (losses)	(208)		(518)
Investments at fair value, end of period	1,713		1,534
Fair Value, Measurements, Recurring | Level 3 | Operating Companies | Warrants and Rights, sold not yet purchased [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Investments at fair value, beginning of period	0		0
Transfers in	0		0
Transfers out	(1,004)	[2]	0
Purchases/(covers)	(297)		0
(Sales)/short buys	977		0
Realized gains (losses)	(37)		0
Unrealized gains (losses)	364		0
Investments at fair value, end of period	3		0
Fair Value, Measurements, Recurring | Level 3 | Operating Companies | Common Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Investments at fair value, beginning of period	819		334
Transfers in	0		0
Transfers out	0		0
Purchases/(covers)	1,789		437
(Sales)/short buys	(6)		(568)
Realized gains (losses)	6		159
Unrealized gains (losses)	(59)		457
Investments at fair value, end of period	2,549		819
Fair Value, Measurements, Recurring | Level 3 | Operating Companies | Restricted Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Investments at fair value, beginning of period	0		5,000
Transfers in	0		0
Transfers out	0		0
Purchases/(covers)	0		0
(Sales)/short buys	0		(4,857)
Realized gains (losses)	0		(143)
Unrealized gains (losses)	0		0
Investments at fair value, end of period	0		0
Fair Value, Measurements, Recurring | Level 3 | Operating Companies | Warrants and Rights
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Investments at fair value, end of period	1,713		1,534
Fair Value, Measurements, Recurring | Level 3 | Operating Companies | Portfolio Funds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Investments at fair value, beginning of period	16,919		17,081
Transfers in	0		566	[3]
Transfers out	0		0
Purchases/(covers)	10,116		45,925
(Sales)/short buys	(3,482)		(48,835)
Realized gains (losses)	(41)		157
Unrealized gains (losses)	2,158		2,025
Investments at fair value, end of period	25,670		16,919
Fair Value, Measurements, Recurring | Level 3 | Operating Companies | Real Estate Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Investments at fair value, beginning of period	2,353		1,882
Transfers in	0		0
Transfers out	0		0
Purchases/(covers)	153		330
(Sales)/short buys	(781)		(10)
Realized gains (losses)	0		0
Unrealized gains (losses)	139		151
Investments at fair value, end of period	1,864		2,353
Fair Value, Measurements, Recurring | Level 3 | Operating Companies | Lehman Claim
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Investments at fair value, beginning of period	553		313
Transfers in	0		0
Transfers out	0		0
Purchases/(covers)	0		0
(Sales)/short buys	(234)		0
Realized gains (losses)	0		0
Unrealized gains (losses)	387		240
Investments at fair value, end of period	706		553
Fair Value, Measurements, Recurring | Level 3 | Consolidated Funds | Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Investments at fair value, end of period			0
Fair Value, Measurements, Recurring | Level 3 | Consolidated Funds | Portfolio Funds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Investments at fair value, beginning of period	213,402		311,242
Transfers in	16,227	[4]	0
Transfers out	(17,151)	[4]	0
Purchases/(covers)	434		2
(Sales)/short buys	(28,892)		(104,243)
Realized gains (losses)	(3,823)		2,508
Unrealized gains (losses)	2,723		3,893
Investments at fair value, end of period	182,920		213,402
Fair Value, Measurements, Recurring | Level 3 | Consolidated Funds | Lehman Claim
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Investments at fair value, beginning of period	7,340		6,243
Transfers in	0		0
Transfers out	0		0
Purchases/(covers)	0		0
(Sales)/short buys	(2,292)		0
Realized gains (losses)	1,914		0
Unrealized gains (losses)	7,162		1,097
Investments at fair value, end of period	$ 14,124		$ 7,340

[1] The security was listed on an exchange subsequent to a private funding.

[2] The security began trading on an exchange due to a business combination.

[3] Changes in the observability of inputs in the valuation of such assets.

[4] Change in consolidated funds (see Note 3b).