Summary of Significant Accounting Policies (Details 1) - USD ($) $ in Thousands		Dec. 31, 2019	Dec. 31, 2018
Securities and other investments owned:
Total securities and other investments owned		$ 451,551	$ 273,577
Securities sold not yet purchased:
Total securities sold not yet purchased		41,820	37,623
Corporate Bonds [Member]
Securities sold not yet purchased:
Total securities sold not yet purchased		33,436	16,338
Other Fixed Income Securities [Member]
Securities sold not yet purchased:
Total securities sold not yet purchased		3,024	10,155
Equity Securities [Member]
Securities sold not yet purchased:
Total securities sold not yet purchased		5,360	11,130
Equity Securities [Member] | Significant unobservable inputs (Level 3) [Member]
Securities and other investments owned:
Total assets measured at fair value		109,251
Fair Value, Measurements, Recurring [Member]
Securities and other investments owned:
Total securities and other investments owned		423,934	249,840
Investment funds valued at net asset value	[1]	27,617	23,737
Total assets measured at fair value		451,551	273,577
Securities sold not yet purchased:
Total securities sold not yet purchased		41,820	37,623
Mandatorily redeemable noncontrolling interests issued after November 5, 2003		4,616	4,633
Total liabilities measured at fair value		46,436	42,256
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Securities and other investments owned:
Total securities and other investments owned		243,911	168,882
Securities sold not yet purchased:
Total securities sold not yet purchased		5,360	11,130
Mandatorily redeemable noncontrolling interests issued after November 5, 2003
Total liabilities measured at fair value		5,360	11,130
Fair Value, Measurements, Recurring [Member] | Other observable inputs (Level 2) [Member]
Securities and other investments owned:
Total securities and other investments owned		27,434	22,650
Securities sold not yet purchased:
Total securities sold not yet purchased		36,460	26,493
Mandatorily redeemable noncontrolling interests issued after November 5, 2003
Total liabilities measured at fair value		36,460	26,493
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member]
Securities and other investments owned:
Total securities and other investments owned		43,338	58,308
Securities sold not yet purchased:
Total securities sold not yet purchased
Mandatorily redeemable noncontrolling interests issued after November 5, 2003		4,616	4,633
Total liabilities measured at fair value		4,616	4,633
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Securities and other investments owned:
Total securities and other investments owned		19,020	18,825
Securities sold not yet purchased:
Total securities sold not yet purchased		33,436	16,338
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Securities and other investments owned:
Total securities and other investments owned
Securities sold not yet purchased:
Total securities sold not yet purchased
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member] | Other observable inputs (Level 2) [Member]
Securities and other investments owned:
Total securities and other investments owned		19,020	18,825
Securities sold not yet purchased:
Total securities sold not yet purchased		33,436	16,338
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member] | Significant unobservable inputs (Level 3) [Member]
Securities and other investments owned:
Total securities and other investments owned
Securities sold not yet purchased:
Total securities sold not yet purchased
Fair Value, Measurements, Recurring [Member] | Other Fixed Income Securities [Member]
Securities and other investments owned:
Total securities and other investments owned		8,414	3,825
Securities sold not yet purchased:
Total securities sold not yet purchased		3,024	10,155
Fair Value, Measurements, Recurring [Member] | Other Fixed Income Securities [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Securities and other investments owned:
Total securities and other investments owned
Securities sold not yet purchased:
Total securities sold not yet purchased
Fair Value, Measurements, Recurring [Member] | Other Fixed Income Securities [Member] | Other observable inputs (Level 2) [Member]
Securities and other investments owned:
Total securities and other investments owned		8,414	3,825
Securities sold not yet purchased:
Total securities sold not yet purchased		3,024	10,155
Fair Value, Measurements, Recurring [Member] | Other Fixed Income Securities [Member] | Significant unobservable inputs (Level 3) [Member]
Securities and other investments owned:
Total securities and other investments owned
Securities sold not yet purchased:
Total securities sold not yet purchased
Fair Value, Measurements, Recurring [Member] | Loans Receivable At Fair Value [Member]
Securities and other investments owned:
Total securities and other investments owned		43,338	33,731
Fair Value, Measurements, Recurring [Member] | Loans Receivable At Fair Value [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Securities and other investments owned:
Total securities and other investments owned
Fair Value, Measurements, Recurring [Member] | Loans Receivable At Fair Value [Member] | Other observable inputs (Level 2) [Member]
Securities and other investments owned:
Total securities and other investments owned
Fair Value, Measurements, Recurring [Member] | Loans Receivable At Fair Value [Member] | Significant unobservable inputs (Level 3) [Member]
Securities and other investments owned:
Total securities and other investments owned		43,338	33,731
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member]
Securities and other investments owned:
Total securities and other investments owned		353,162	193,459
Securities sold not yet purchased:
Total securities sold not yet purchased		5,360	11,130
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Securities and other investments owned:
Total securities and other investments owned		243,911	168,882
Securities sold not yet purchased:
Total securities sold not yet purchased		5,360	11,130
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Other observable inputs (Level 2) [Member]
Securities and other investments owned:
Total securities and other investments owned
Securities sold not yet purchased:
Total securities sold not yet purchased
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Significant unobservable inputs (Level 3) [Member]
Securities and other investments owned:
Total securities and other investments owned		109,251	24,577
Securities sold not yet purchased:
Total securities sold not yet purchased

[1] Certain investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy in accordance with ASC "Topic 820 Fair Value Measurements." The fair value amounts presented in the tables above for investment funds valued at net asset value are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the consolidated balance sheets.