Investments of Operating Entities and Consolidated Funds - Securities Sold, Not Yet Purchased (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013
Investments Sold, Not yet Purchased [Line Items]
Securities owned, at fair value	$ 580,963		$ 311,039
Securities sold, not yet purchased, at fair value	235,592		130,954
Merger Master
Investments Sold, Not yet Purchased [Line Items]
Securities owned, at fair value	99,137		48,545
Corporate Bonds
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	58	[1]	55	[1]
Securities sold, not yet purchased, interest rate	5.55%		5.55%
Common Stock
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	235,534		130,899
Common Stock | Merger Master
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	48,100		19,500
Options | Merger Master
Investments Sold, Not yet Purchased [Line Items]
Securities owned, at fair value	1,411		200
Corporate Bond Securities [Member] | Merger Master
Investments Sold, Not yet Purchased [Line Items]
Securities owned, at fair value	16,059	[2]	14,444	[2]
Common Stock | Merger Master
Investments Sold, Not yet Purchased [Line Items]
Securities owned, at fair value	$ 81,667		$ 33,901

[1] As of June 30, 2014 and December 31, 2013, the maturity was January 2026 with an interest rate of 5.55%

[2] As of June 30, 2014, maturities ranged from February 2017 to October 2022 and interest rates ranged between 5.38% and 9.75%. As of December 31, 2013, maturities ranged from April 2016 to October 2020 and interest rates ranged between 7.00% and 10.88%.