Investments of Operating Entities and Consolidated Funds - Securities Sold, Not Yet Purchased (Details) - USD ($)		9 Months Ended		12 Months Ended
		Sep. 30, 2017		Dec. 31, 2016
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value, Consolidated Funds		$ 0	[1]	$ 883,000
Securities sold, not yet purchased, at fair value		$ 305,267,000		$ 266,090,000
Convertible Debt Securities [Member]
Investments Sold, Not yet Purchased [Line Items]
Investment Interest Rate				8.00%
Convertible Debt Securities [Member] | Minimum
Investments Sold, Not yet Purchased [Line Items]
Investment Interest Rate		5.00%
Convertible Debt Securities [Member] | Maximum
Investments Sold, Not yet Purchased [Line Items]
Investment Interest Rate		12.00%
Corporate Bonds
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value, Consolidated Funds	[1]			$ 672,000
Securities sold, not yet purchased, at fair value	[2]	$ 59,000		$ 2,591,000
Securities sold, not yet purchased, interest rate		5.55%
Corporate Bonds | Minimum
Investments Sold, Not yet Purchased [Line Items]
Investment Interest Rate		0.00%		6.25%
Securities sold, not yet purchased, interest rate				5.50%
Corporate Bonds | Maximum
Investments Sold, Not yet Purchased [Line Items]
Investment Interest Rate		10.75%		13.00%
Securities sold, not yet purchased, interest rate				6.25%
Corporate Bonds | Consolidated Funds | Minimum
Investments Sold, Not yet Purchased [Line Items]
Investment Interest Rate				0.00%
Securities sold, not yet purchased, interest rate				4.38%
Corporate Bonds | Consolidated Funds | Maximum
Investments Sold, Not yet Purchased [Line Items]
Investment Interest Rate				14.37%
Securities sold, not yet purchased, interest rate				9.25%
Common Stock
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value, Consolidated Funds				$ 211,000
Securities sold, not yet purchased, at fair value		$ 305,022,000		263,460,000
Common Stock | Merger Master
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		451,700,000		395,500,000
Common Stock | Caerus Master [Member]
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value				2,600,000
Warrants and Rights
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		$ 186,000		$ 39,000

[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 September 30, 2017, the maturity was January 2026 with an interest rate of 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%.