Revenue Recognition (Disaggregation of revenue) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Total Revenue from Principal transactions	$ 2,041,000	$ 2,634,000	$ 5,479,000	$ 7,838,000
Total Revenue from Commissions and fees	1,925,000	2,347,000	6,030,000	7,380,000
Total Revenue from Margin interest, marketing and distribution fees	2,944,000	2,731,000	8,499,000	7,953,000
Total Revenue from Advisory fees	211,000	138,000	572,000	309,000
Total Revenue from Interest	23,000	34,000	54,000	69,000
Total Revenue	7,144,000	7,884,000	20,634,000	23,549,000
Legacy Siebert [Member]
Disaggregation of Revenue [Line Items]
Total Revenue from Advisory fees	211,000	138,000	572,000	309,000
Total Revenue from Interest	23,000	34,000	54,000	69,000
Margin interest, marketing and distribution fees [Member] | Legacy Siebert [Member]
Disaggregation of Revenue [Line Items]
Total Revenue from Margin interest, marketing and distribution fees	2,566,000	2,409,000	7,451,000	7,049,000
Margin interest, marketing and distribution fees [Member] | StockCross accounts [Member]
Disaggregation of Revenue [Line Items]
Total Revenue from Margin interest, marketing and distribution fees	378,000	322,000	1,048,000	904,000
Commissions and fees [Member] | Legacy Siebert [Member]
Disaggregation of Revenue [Line Items]
Total Revenue from Commissions and fees	1,544,000	1,950,000	5,043,000	5,914,000
Commissions and fees [Member] | StockCross accounts [Member]
Disaggregation of Revenue [Line Items]
Total Revenue from Commissions and fees	381,000	397,000	987,000	1,466,000
Principal transactions [Member] | Legacy Siebert [Member]
Disaggregation of Revenue [Line Items]
Total Revenue from Principal transactions	582,000	437,000	1,496,000	1,514,000
Principal transactions [Member] | StockCross accounts [Member]
Disaggregation of Revenue [Line Items]
Total Revenue from Principal transactions	$ 1,459,000	$ 2,197,000	$ 3,983,000	$ 6,324,000