Investments of Operating Entities and Consolidated Funds - Securities Sold, Not Yet Purchased (Details) - USD ($) $ in Thousands		Sep. 30, 2015	Dec. 31, 2014
Investments Sold, Not yet Purchased [Line Items]
Receivable on derivative contracts, at fair value		$ 41,587	$ 49,877
Securities owned, at fair value		864,685	792,206
Securities sold, not yet purchased, at fair value		250,739	207,875
Merger Master
Investments Sold, Not yet Purchased [Line Items]
Receivable on derivative contracts, at fair value		776	617
Securities owned, at fair value		179,650	136,893
Corporate Bonds
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	[1]	$ 59	$ 60
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		$ 250,680	$ 207,815
Common Stock | Merger Master
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		57,700	39,900
Currency forward | Merger Master
Investments Sold, Not yet Purchased [Line Items]
Receivable on derivative contracts, at fair value		22	0
Options | Merger Master
Investments Sold, Not yet Purchased [Line Items]
Receivable on derivative contracts, at fair value		604	541
Equity Swaps | Merger Master
Investments Sold, Not yet Purchased [Line Items]
Receivable on derivative contracts, at fair value		$ 150	$ 76

[1] As of September 30, 2015 and December 31, 2014, the maturity was January 2026 with an interest rate of 5.55%