Revenue Recognition (Schedule of Major Revenue Categories) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Trading Execution and Clearing Services
Commissions and fees	$ 4,887,000	$ 2,591,000	$ 10,470,000	$ 4,859,000
Principal transactions	2,581,000	1,921,000	5,784,000	3,811,000
Market making	615,000	410,000	1,085,000	973,000
Stock borrow / stock loan	771,000	423,000	1,215,000	1,004,000
Advisory fees	243,000	193,000	505,000	361,000
Total Trading Execution and Clearing Services	9,097,000	5,538,000	19,059,000	11,008,000
Margin interest, marketing and distribution fees
Margin interest	1,829,000	2,870,000	4,335,000	5,687,000
12b1 fees	296,000	823,000	1,084,000	1,562,000
Total Margin interest, marketing and distribution fees	2,125,000	3,693,000	5,419,000	7,249,000
Interest income	909,000	1,186,000	2,240,000	2,359,000
Other income	488,000	264,000	702,000	343,000
Total Other Income	3,522,000	5,143,000	8,361,000	9,951,000
Total Revenue	$ 12,619,000	$ 10,681,000	$ 27,420,000	$ 20,959,000