Investments of Operating Entities and Consolidated Funds - Securities Sold, Not Yet Purchased (Details) - USD ($)		Jun. 30, 2019	Dec. 31, 2018
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		$ 438,584,000	$ 195,307,000
Common Stock
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		409,034,000	194,305,000
Corporate Bonds
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	[1]	$ 2,581,000	$ 750,000
Corporate Bonds | Minimum
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate		1.88%	2.25%
Corporate Bonds | Maximum
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate		15.50%	9.38%
Government Securities | Minimum
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate		7.00%
Government Securities | Maximum
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate		8.25%
US Government Debt Securities
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	[2]	$ 4,681,000	$ 0
Preferred Stock
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		2,088,000	199,000
Warrants and Rights
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		$ 20,200,000	$ 53,000

[1] As of June 30, 2019, the maturities ranged from October 2020 to May 2029 and interest rates ranged from 1.88% to 15.50%. As of December 31, 2018, the maturities ranged from October 2022 to January 2034 and interest rates ranged from 2.25% to 9.38%.

[2] As of June 30, 2019, the maturities ranged from October 2019 to March 2038 and interest rates ranged from 7.00% to 8.25%.