Investments of Operating Entities and Consolidated Funds - Securities Sold, Not Yet Purchased (Details) - USD ($) $ in Thousands		Mar. 31, 2016	Dec. 31, 2015
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, Consolidated Funds		$ 1,367	$ 0
Securities owned, at fair value		627,767	610,234
Securities sold, not yet purchased, at fair value		412,810	257,159
Merger Master
Investments Sold, Not yet Purchased [Line Items]
Securities owned, at fair value		814,309	157,921
Corporate Bonds
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	[1]	$ 60	$ 58
Securities sold, not yet purchased, interest rate		5.55%	5.55%
Corporate Bonds | Consolidated Funds | Minimum
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate		4.38%
Corporate Bonds | Consolidated Funds | Maximum
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate		9.25%
Common Stock
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		$ 412,750	$ 257,101
Common Stock | Merger Master
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		581,300	73,800
Corporate Bonds
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, Consolidated Funds	[2]	1,015
Corporate Bonds | Merger Master
Investments Sold, Not yet Purchased [Line Items]
Securities owned, at fair value		0	492	[3]
Common Stock
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, Consolidated Funds		352
Common Stock | Merger Master
Investments Sold, Not yet Purchased [Line Items]
Securities owned, at fair value		$ 814,309	$ 157,429

[1] As of March 31, 2016 and December 31, 2015, the maturity was January 2026 with an interest rate of 5.55%

[2] As of March 31, 2016, maturities ranged from September 2019 to March 2025 and interest rates ranged between 4.38% and 9.25%.

[3] As of December 31, 2015, the maturity was June 2024 with an interest rate of 5.25%.