Summary of Significant Accounting Policies (Details) - Schedule of securities and other investments owned and securities sold not yet purchased at fair value - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Securities and other investments owned:
Securities and other investments owned	$ 459,480	$ 408,213
Securities sold not yet purchased:
Securities sold not yet purchased	48,125	41,820
Corporate Bonds [Member]
Securities and other investments owned:
Securities and other investments owned	5,956	19,020
Securities sold not yet purchased:
Securities sold not yet purchased	4,490	33,436
Other Fixed Income Securities [Member]
Securities and other investments owned:
Securities and other investments owned	3,557	8,414
Securities sold not yet purchased:
Securities sold not yet purchased	1,549	3,024
Partnership Interests and Other [Member]
Securities and other investments owned:
Securities and other investments owned	57,293	27,617
Equity Securities [Member]
Securities and other investments owned:
Securities and other investments owned	392,674	353,162
Securities sold not yet purchased:
Securities sold not yet purchased	$ 42,086	$ 5,360