Revenue Recognition (Schedule of Major Revenue Categories) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Trading Execution and Clearing Services
Commissions and fees	$ 4,325,000	$ 4,887,000	$ 11,333,000	$ 10,470,000
Principal transactions	4,107,000	2,581,000	8,355,000	5,784,000
Market making	1,758,000	615,000	3,372,000	1,085,000
Stock borrow / stock loan	2,240,000	771,000	4,087,000	1,215,000
Advisory fees	403,000	243,000	759,000	505,000
Total Trading Execution and Clearing Services	12,833,000	9,097,000	27,906,000	19,059,000
Other Income
Interest	1,163,000	909,000	2,317,000	2,240,000
Margin interest	2,296,000	1,829,000	4,448,000	4,335,000
12b-1 fees	164,000	296,000	317,000	1,084,000
Total Interest, marketing and distribution fees	3,623,000	3,034,000	7,082,000	7,659,000
Other income	337,000	488,000	729,000	702,000
Total Revenue	$ 16,793,000	$ 12,619,000	$ 35,717,000	$ 27,420,000