Revenue Recognition (Disaggregation of revenue) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Total Revenue from Principal transactions	$ 1,766,000	$ 2,263,000	$ 3,438,000	$ 5,204,000
Total Revenue from Commissions and fees	2,303,000	2,376,000	4,105,000	5,033,000
Total Revenue from Margin interest, marketing and distribution fees	2,783,000	2,687,000	5,555,000	5,222,000
Total Revenue from Advisory fees	193,000	138,000	361,000	171,000
Total Revenue from Interest	16,000	24,000	31,000	35,000
Total Revenue	7,061,000	7,488,000	13,490,000	15,665,000
Legacy Siebert [Member]
Disaggregation of Revenue [Line Items]
Total Revenue from Advisory fees	193,000	138,000	361,000	171,000
Total Revenue from Interest	16,000	24,000	31,000	35,000
Margin interest, marketing and distribution fees [Member] | Legacy Siebert [Member]
Disaggregation of Revenue [Line Items]
Total Revenue from Margin interest, marketing and distribution fees	2,440,000	2,312,000	4,885,000	4,640,000
Margin interest, marketing and distribution fees [Member] | StockCross accounts [Member]
Disaggregation of Revenue [Line Items]
Total Revenue from Margin interest, marketing and distribution fees	343,000	375,000	670,000	582,000
Commissions and fees [Member] | Legacy Siebert [Member]
Disaggregation of Revenue [Line Items]
Total Revenue from Commissions and fees	1,981,000	2,026,000	3,499,000	3,964,000
Commissions and fees [Member] | StockCross accounts [Member]
Disaggregation of Revenue [Line Items]
Total Revenue from Commissions and fees	322,000	350,000	606,000	1,069,000
Principal transactions [Member] | Legacy Siebert [Member]
Disaggregation of Revenue [Line Items]
Total Revenue from Principal transactions	474,000	505,000	915,000	1,077,000
Principal transactions [Member] | StockCross accounts [Member]
Disaggregation of Revenue [Line Items]
Total Revenue from Principal transactions	$ 1,292,000	$ 1,758,000	$ 2,523,000	$ 4,127,000