Fair Value Measurements for Operating Entities and Consolidated Funds Assets and Liabilities at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2015		Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	$ 927,523		$ 845,131
Financial Liabilities Fair Value Disclosure	282,275		253,288
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, Low	0		900
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High	6,700		7,100
Other investments	153,334		167,464
Other Investments, Consolidated Funds	255,896		189,377
Investments	1,334,502		1,198,924
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	718,314		607,352
Financial Liabilities Fair Value Disclosure	252,445		210,735
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	146,156		182,773
Financial Liabilities Fair Value Disclosure	2,781		1,663
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	63,053		55,006
Financial Liabilities Fair Value Disclosure	27,049		40,890
Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	250,680		207,815
Common Stock | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	250,680		207,815
Common Stock | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	0		0
Common Stock | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	0		0
Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	59		60
Corporate Bonds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	0		0
Corporate Bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	59		60
Corporate Bonds | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	0		0
Contingent liability payable
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	5,458	[1]	4,083	[2]
Contingent liability payable | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	0	[1]	0	[2]
Contingent liability payable | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	0	[1]	0	[2]
Contingent liability payable | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	5,458	[1]	4,083	[2]
Futures | Derivative Liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	381		33
Futures | Derivative Liabilities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	381		33
Futures | Derivative Liabilities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	0		0
Futures | Derivative Liabilities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	0		0
Currency forward | Derivative Liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	$ 107
Currency forward | Derivative Liabilities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure
Currency forward | Derivative Liabilities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	$ 107
Currency forward | Derivative Liabilities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	0
Equity Swaps | Derivative Liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	$ 2,615		1,603
Equity Swaps | Derivative Liabilities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure			0
Equity Swaps | Derivative Liabilities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	$ 2,615		1,603
Equity Swaps | Derivative Liabilities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	0		0
Put Option | Derivative Liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	22,975		39,694
Put Option | Derivative Liabilities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	1,384		2,887
Put Option | Derivative Liabilities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	0		0
Put Option | Derivative Liabilities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	21,591		36,807
US Government Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	3,015		2,010
US Government Securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	3,015		2,010
US Government Securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0		0
US Government Securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0		0
Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	15,771		15,070
Preferred Stock | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0		0
Preferred Stock | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0		2,553
Preferred Stock | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	15,771		12,517
Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	712,489		597,476
Common Stock | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	689,252		578,934
Common Stock | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	22,920		18,130
Common Stock | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	317		412
Convertible Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	1,129		900
Convertible Bonds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0		0
Convertible Bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0		0
Convertible Bonds | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	1,129		900
Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	113,585		159,557
Corporate Bonds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0		0
Corporate Bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	113,585		159,557
Corporate Bonds | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0		0
Warrants and Rights
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	3,871		1,417
Warrants and Rights | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	877		95
Warrants and Rights | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0		0
Warrants and Rights | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	2,994		1,322
Mutual Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	14,825		15,776
Mutual Funds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	14,825		15,776
Mutual Funds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0		0
Mutual Funds | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0		0
Derivative Assets | Futures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	68		75
Derivative Assets | Futures | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	68		75
Derivative Assets | Futures | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0		0
Derivative Assets | Futures | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0		0
Derivative Assets | Currency forward
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	105		310
Derivative Assets | Currency forward | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0		0
Derivative Assets | Currency forward | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	105		310
Derivative Assets | Currency forward | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0		0
Derivative Assets | Equity Swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	470		251
Derivative Assets | Equity Swaps | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0		0
Derivative Assets | Equity Swaps | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	470		251
Derivative Assets | Equity Swaps | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0		0
Derivative Assets | Options
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	40,944		49,241
Derivative Assets | Options | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	10,277		10,462
Derivative Assets | Options | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	9,076		1,972
Derivative Assets | Options | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	21,591		36,807
Portfolio Funds, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	108,509	[3]	103,466	[4]
Other Investments, Consolidated Funds	255,896	[3]	188,884	[4]
Real Estate Investments, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	1,916		2,175
Real Estate Investments, at fair value | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0		0
Real Estate Investments, at fair value | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0		0
Real Estate Investments, at fair value | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	1,916		2,175
Lehman claims, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	335		380
Other investments	335		380
Other Investments, Consolidated Funds	0		493
Lehman claims, at fair value | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0		0
Lehman claims, at fair value | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0		0
Lehman claims, at fair value | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	335		380
Equity Method Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	42,574		61,443
Consolidated Funds | Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	19,000
Consolidated Funds | Preferred Stock | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0
Consolidated Funds | Preferred Stock | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0
Consolidated Funds | Preferred Stock | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	$ 19,000
Consolidated Funds | Lehman claims, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring			493
Consolidated Funds | Lehman claims, at fair value | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring			0
Consolidated Funds | Lehman claims, at fair value | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring			0
Consolidated Funds | Lehman claims, at fair value | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring			$ 493

[1] In accordance with the terms of the purchase agreements for acquisitions that closed during 2012 and the third quarter of 2015, the Company is required to pay to the sellers a portion of future net income of the acquired businesses, if certain revenue targets are achieved through the periods ended August 2016 and December 2020, respectively. The Company estimated the contingent consideration liability using the income approach (discounted cash flow method) which requires the Company to make estimates and assumptions regarding the future cash flows and profits. Changes in these estimates and assumptions could have a significant impact on the amounts recognized. The undiscounted amounts as of September 30, 2015 can range from zero to $6.7 million.

[2] In accordance with the terms of a purchase agreement for an acquisitions that closed during 2012, the Company is required to pay to the sellers a portion of future net income of the acquired businesses, if certain revenue targets are achieved through the period ended August 2016. The Company estimated the contingent consideration liability using the income approach (discounted cash flow method) which requires the Company to make estimates and assumptions regarding the future cash flows and profits. Changes in these estimates and assumptions could have a significant impact on the amounts recognized. The undiscounted amounts can range from $0.9 million to $7.1 million.

[3] In accordance with US GAAP, certain investments that are measured at fair value using the net asset value per share (or its equivalent) as a practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the condensed consolidated statement of financial condition.

[4] In accordance with US GAAP, certain investments that are measured at fair value using the net asset value per share (or its equivalent) as a practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the condensed consolidated statement of financial condition.