Acquisitions (Narrative) (Details) - USD ($)			1 Months Ended	12 Months Ended
	Dec. 02, 2019	Sep. 05, 2019	Jan. 31, 2019	Dec. 31, 2020	Dec. 31, 2019		Jan. 02, 2020
Business Acquisition [Line Items]
Purchase of StockCross common stock					$ 3,424,000
Treasury stock sales - StockCross					172,000
Goodwill				1,989,000	1,989,000	[1]
Return of capital distribution					1,600,000
Unrelated party [Member]
Business Acquisition [Line Items]
Purchase of StockCross common stock					$ 3,665,000
StockCross [Member]
Business Acquisition [Line Items]
Ownership percentage acquired			15.00%		15.00%
Treasury stock sales - StockCross					$ 172,000
Purchase price			$ 29,750,000
Purchase price in shares as restricted stock			3,298,774
Goodwill
Return of capital distribution		$ 1,600,000
Proceeds from return of captial		$ 241,000
When StockCross Merged with MSCO [Member]
Business Acquisition [Line Items]
Ownership percentage acquired							85.00%
WP [Member]
Business Acquisition [Line Items]
Ownership percentage acquired	100.00%
Purchase price	$ 7,100,000
Estimated effective tax rate	28.00%
Promissory note	$ 3,000,000
Goodwill	$ 1,989,000

[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.