Revenue Recognition (Schedule of Major Revenue Categories) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Trading Execution and Clearing Services
Commissions and fees	$ 5,583,000	$ 2,268,000
Principal transactions	3,203,000	1,890,000
Market making	470,000	563,000
Stock borrow / stock loan	444,000	581,000
Advisory fees	262,000	168,000
Total Trading Execution and Clearing Services	9,962,000	5,470,000
Margin interest, marketing and distribution fees
Margin interest	2,506,000	2,817,000
12b1 fees	788,000	739,000
Total Margin interest, marketing and distribution fees	3,294,000	3,556,000
Interest income	1,331,000	1,173,000
Other income	214,000	79,000
Total Other Income	4,839,000	4,808,000
Total Revenue	$ 14,801,000	$ 10,278,000