Goodwill and Intangible Assets, Net (Narrative) (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019	[1]	Dec. 01, 2019
Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 1,989,000	$ 1,989,000
Customer Relationships [Member] | Weeden Prime Services LLC [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible assets - initial value				$ 987,000
Trade Names [Member] | Weeden Prime Services LLC [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible assets - initial value				$ 70,000

[1] Statement of financial condition as of December 31, 2019 represents the pro forma combination of Siebert and StockCross balances. See "Note 3 - Acquisitions" for additional detail.