Revenue Recognition (Schedule of Major Revenue Categories) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Trading Execution and Clearing Services
Commissions and fees	$ 2,340,000	$ 7,008,000
Principal transactions and proprietary trading	(267,000)	4,248,000
Market making	764,000	1,614,000
Stock borrow / stock loan	3,578,000	1,847,000
Advisory fees	507,000	356,000
Total Trading Execution and Clearing Services	6,922,000	15,073,000
Other Income
Interest	235,000	1,154,000
Margin interest	1,968,000	2,152,000
12b-1 fees	159,000	153,000
Total Interest, marketing and distribution fees	2,362,000	3,459,000
Other income	1,060,000	392,000
Total Revenue	$ 10,344,000	$ 18,924,000