Summary of Significant Accounting Policies (Schedule of Disaggregation of Revenue) (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
Revenue from Principal transactions:
Principal transactions - Legacy Siebert									$ 1,894,000	$ 490,000
Principal transactions - StockCross Retail Assets									7,126,000	1,149,000
Total Revenue from Principal transactions									9,020,000	1,639,000
Revenue from Commissions and fees:
Commissions and fees - Legacy Siebert									7,792,000	4,527,000
Commissions and fees - StockCross Retail Assets									1,712,000	274,000
Total Revenue from Commissions and fees									9,504,000	4,801,000
Revenue from Margin interest, marketing and distribution fees:
Margin interest, marketing and distribution fees - Legacy Siebert									9,674,000	6,409,000
Margin interest, marketing and distribution fees - StockCross Retail Assets									1,254,000	191,000
Total Revenue from Margin interest, marketing and distribution fees									10,928,000	6,600,000
Additional Revenue:
Advisory fees - Legacy Siebert									478,000	51,000
Interest - Legacy Siebert									106,000	19,000
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