Revenue Recognition (Schedule of Major Revenue Categories) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Trading Execution and Clearing Services
Commissions and fees	$ 7,008,000	$ 5,583,000
Principal transactions	4,248,000	3,203,000
Market making	1,614,000	470,000
Stock borrow / stock loan	1,847,000	444,000
Advisory fees	356,000	262,000
Total Trading Execution and Clearing Services	15,073,000	9,962,000
Other Income
Interest	1,154,000	1,331,000
Margin interest	2,152,000	2,506,000
12b-1 fees	153,000	788,000
Total Interest, marketing and distribution fees	3,459,000	4,625,000
Other income	392,000	214,000
Total Revenue	$ 18,924,000	$ 14,801,000