Summary of Significant Accounting Policies (Schedule of Major Revenue Categories) (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Trading Execution and Clearing Services
Commissions and fees									$ 9,504,000	$ 4,801,000
Principal transactions									9,020,000	1,639,000
Advisory fees and additional income									584,000	70,000
Margin interest, marketing and distribution fees
Margin interest									7,663,000	3,726,000
12b1 fees									3,265,000	2,874,000
Total Margin interest, marketing and distribution fees									10,928,000	6,600,000
Total Revenue	$ 6,487,000	$ 7,884,000	$ 7,488,000	$ 8,177,000	$ 4,953,000	$ 3,089,000	$ 2,689,000	$ 2,379,000	$ 30,036,000	$ 13,110,000