Investments of Operating Entities and Consolidated Funds - Securities Sold, Not Yet Purchased (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Investments Sold, Not yet Purchased [Line Items]
Receivable on derivative contracts, at fair value	$ 56,953		$ 49,877
Securities owned, at fair value	579,458		792,206
Securities sold, not yet purchased, at fair value	163,492		207,875
Merger Master
Investments Sold, Not yet Purchased [Line Items]
Receivable on derivative contracts, at fair value	70		78
Securities owned, at fair value	134,717		137,434
Options | Merger Master
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	200		200
Corporate Bonds
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	60	[1]	60	[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	163,432		207,815
Common Stock | Merger Master
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	28,800		39,900
Equity Swaps | Merger Master
Investments Sold, Not yet Purchased [Line Items]
Receivable on derivative contracts, at fair value	70		78
Options | Merger Master
Investments Sold, Not yet Purchased [Line Items]
Securities owned, at fair value	992		541
Corporate Bonds | Merger Master
Investments Sold, Not yet Purchased [Line Items]
Securities owned, at fair value	0	[2]	3,383	[2]
Common Stock | Merger Master
Investments Sold, Not yet Purchased [Line Items]
Securities owned, at fair value	$ 133,725		$ 133,510

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

[2] As of December 31, 2014, maturities ranged from February 2017 to June 2019 and interest rates ranged between 8.50% and 9.75%.