Investments of Operating Entities and Consolidated Funds - Merger Master Narrative (Details) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Trading Securities		$ 543,740	$ 673,221
Securities sold, not yet purchased, at fair value		261,586	342,527
Common Stock
Investment [Line Items]
Securities sold, not yet purchased, at fair value		252,585	342,328
Common Stock | Merger Master [Member]
Investment [Line Items]
Trading Securities		902,800	483,200
Securities sold, not yet purchased, at fair value		230,100	171,200
Corporate Bonds
Investment [Line Items]
Securities sold, not yet purchased, at fair value	[1]	1,287	$ 122
Corporate Bonds | Merger Master [Member]
Investment [Line Items]
Trading Securities		$ 38,000

[1] As of March 31, 2018, the maturities ranged from August 2018 to January 2026 with interest rates ranged from 5.55% to 7.50%. As of December 31, 2017, the maturities ranged from July 2025 to January 2026 with interest rates ranged from 3.78% to 5.55%.