Investments of Operating Entities and Consolidated Funds - Securities Sold, Not Yet Purchased (Details) - USD ($) $ in Thousands		Jun. 30, 2016	Dec. 31, 2015
Investments Sold, Not yet Purchased [Line Items]
Portfolio Funds, Consolidated Funds		$ 374,683	$ 263,818
Securities sold, not yet purchased, Consolidated Funds		1,553	0
Securities sold, not yet purchased, at fair value		285,414	257,159
Corporate Bond Securities [Member]
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, Consolidated Funds	[1]	1,195
Securities sold, not yet purchased, at fair value	[2]	$ 1,131	$ 58
Securities sold, not yet purchased, interest rate		5.55%	5.55%
Corporate Bond Securities [Member] | Minimum
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate			5.55%
Corporate Bond Securities [Member] | Maximum
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate			8.25%
Corporate Bond Securities [Member] | Consolidated Funds | Minimum
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate		4.38%
Corporate Bond Securities [Member] | 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, Consolidated Funds		$ 358
Securities sold, not yet purchased, at fair value		284,283	$ 257,101
Common Stock | Merger Master
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		325,100	$ 73,800
Common Stock | Caerus Master [Member]
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		$ 4,800

[1] As of June 30, 2016, maturities ranged from September 2019 to March 2025 and interest rates ranged between 4.38% and 9.25%.

[2] As of June 30, 2016 and December 31, 2015, the maturities ranged from February 2020 to January 2026 with interest rates ranged between 5.55%