Investments of Operating Entities and Consolidated Funds - Merger Master Narrative (Details) - USD ($) $ in Thousands		Dec. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Trading securities		$ 520,888	$ 673,221
Securities sold, not yet purchased, at fair value		195,307	342,527
Common Stock
Investment [Line Items]
Securities sold, not yet purchased, at fair value		194,305	342,328
Common Stock | Merger Master
Investment [Line Items]
Trading securities		162,800	483,200
Securities sold, not yet purchased, at fair value		9,600	171,200
Corporate Bonds
Investment [Line Items]
Securities sold, not yet purchased, at fair value	[1]	750	$ 122
Corporate Bonds | Merger Master
Investment [Line Items]
Trading securities		$ 116,500

[1] As of December 31, 2018, the maturities ranged from October 2022 to January 2034 with interest rates ranged from 2.25% to 9.38%. As of December 31, 2017, the maturities ranged from July 2025 to January 2026 with interest rates ranged from 3.78% to 5.55%.