Investments of Operating Entities and Consolidated Funds - Merger Master Narrative (Details) - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
Investment [Line Items]
Trading securities		$ 644,169	$ 673,221
Securities sold, not yet purchased, at fair value		374,518	342,527
Common Stock
Investment [Line Items]
Securities sold, not yet purchased, at fair value		353,113	342,328
Common Stock | Merger Master
Investment [Line Items]
Trading securities		342,100	483,200
Securities sold, not yet purchased, at fair value		128,600	171,200
Corporate Bonds
Investment [Line Items]
Securities sold, not yet purchased, at fair value	[1]	20,309	$ 122
Corporate Bonds | Merger Master
Investment [Line Items]
Trading securities		$ 23,800

[1] As of September 30, 2018, the maturities ranged from January 2026 to April 2049 with interest rates ranged from 5.00% to 5.55%. As of December 31, 2017, the maturities ranged from July 2025 to January 2026 with interest rates ranged from 3.78% to 5.55%.