Revenue Recognition (Schedule of Major Revenue Categories) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Trading Execution and Clearing Services
Commissions and fees	$ 4,019,000	$ 4,679,000	$ 15,352,000	$ 15,149,000
Principal transactions	3,924,000	2,342,000	12,279,000	8,126,000
Market making	1,514,000	423,000	4,886,000	1,508,000
Stock borrow / stock loan	3,465,000	1,267,000	7,552,000	2,482,000
Advisory fees	441,000	305,000	1,200,000	810,000
Total Trading Execution and Clearing Services	13,363,000	9,016,000	41,269,000	28,075,000
Other Income
Interest	1,014,000	915,000	3,331,000	3,155,000
Margin interest	2,251,000	2,130,000	6,698,000	6,465,000
12b-1 fees	170,000	181,000	488,000	1,265,000
Total Interest, marketing and distribution fees	3,435,000	3,226,000	10,517,000	10,885,000
Other income	253,000	333,000	982,000	1,035,000
Total Revenue	$ 17,051,000	$ 12,575,000	$ 52,768,000	$ 39,995,000