Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Securities and other investments owned:
Securities and other investments owned	$ 451,551	$ 273,577
Securities sold not yet purchased:
Securities sold not yet purchased	41,820	37,623
Corporate Bonds [Member]
Securities and other investments owned:
Securities and other investments owned	19,020	18,825
Securities sold not yet purchased:
Securities sold not yet purchased	33,436	16,338
Other Fixed Income Securities [Member]
Securities and other investments owned:
Securities and other investments owned	8,414	3,825
Securities sold not yet purchased:
Securities sold not yet purchased	3,024	10,155
Loans Receivable At Fair Value [Member]
Securities and other investments owned:
Securities and other investments owned	43,338	33,731
Partnership Interests and Other [Member]
Securities and other investments owned:
Securities and other investments owned	27,617	23,737
Equity Securities [Member]
Securities and other investments owned:
Securities and other investments owned	353,162	193,459
Securities sold not yet purchased:
Securities sold not yet purchased	$ 5,360	$ 11,130