Investments of Operating Entities and Consolidated Funds - Securities Sold, Not Yet Purchased (Details) - USD ($)		Dec. 31, 2017		Dec. 31, 2016
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		$ 342,527,000		$ 266,090,000
Securities sold, not yet purchased, at fair value, Consolidated Funds		0	[1]	883,000
Common Stock
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		342,328,000		263,460,000
Securities sold, not yet purchased, at fair value, Consolidated Funds				211,000
Corporate Bonds
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	[2]	$ 122,000		2,591,000
Securities sold, not yet purchased, at fair value, Consolidated Funds	[1]			$ 672,000
Corporate Bonds | Minimum
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate		3.78%		5.50%
Corporate Bonds | Maximum
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate		5.55%		6.25%
Preferred Stock
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	[2]	$ 77,000		$ 0
Warrants and Rights
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		$ 0		$ 39,000
Consolidated Funds | Corporate Bonds | Minimum
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate				4.38%
Consolidated Funds | Corporate Bonds | Maximum
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate				9.25%

[1] As of December 31, 2016, maturities ranged from September 2019 to September 2023 and interest rates ranged from 4.38% to 9.25%.

[2] As of December 31, 2017, the maturities ranged from July 2025 to January 2036 with interest rates ranged from 3.78% to 5.55%. As of December 31, 2016, the maturities ranged from April 2021 to January 2036 with interest rates ranged from 5.50% to 6.25%.